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<!-- saved from url=(0064)http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html -->
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<b><font face="Univers">
</font><p align="CENTER"><font face="Univers"></font>PART II</p>
<p align="CENTER">A GUIDE TO THE CODE FORM FM 92-IX Ext. GRIB</p>
<p align="CENTER">Edition 1</p>
</b>
<p align="CENTER">TABLE OF CONTENTS</p>
<p><a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Introduction">Introduction</a><br>
<a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#GRIB">GRIB Code Form</a><br>
<a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Section0">Section 0: The indicator section</a><br>
<a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Section1">Section 1: The product definition section</a><br>
<a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Section2">Section 2: Grid description section <br>
</a><a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Section3">Section 3: Bit map section</a><br>
<a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Section4">Section 4: Binary data section</a><br>
<a href="http://www.wmo.int/pages/prog/www/WDM/Guides/Guide-binary-2.html#Section5">Section 5: End section</a></p>
<p align="JUSTIFY">&nbsp;<b><a name="Introduction">INTRODUCTION</a></b></p><b>
</b>
<p align="JUSTIFY">The World Meteorological Organization (WMO) Commission for Basic
Systems (CBS) Extraordinary Meeting Number VIII (1985) approved a general purpose,
bit-oriented data exchange format, designated FM 92-VIII Ext. GRIB (GRIdded Binary). It is
an efficient vehicle for transmitting large volumes of gridded data to automated centers
over high-speed telecommunication lines using modern protocols. By packing information
into the GRIB code, messages (or records - the terms are synonymous in this context) can
be made more compact than character oriented bulletins, which will produce faster
computer-to-computer transmissions. GRIB can equally well serve as a data storage format,
generating the same efficiencies relative to information storage and retrieval devices.</p>
<p align="JUSTIFY">Changes and extensions to GRIB were approved at the regular meeting of
the WMO/CBS in February 1988; additional changes were introduced at the CBS/WGDM/Sub-Group
on Data Representation (SGDR) meetings in May 1989 and in October 1990. The 1990 changes
were of such structural magnitude as to require a new Edition of GRIB, Edition 1, which
this document describes. The Sub-Group made further augmentations and interpretations on
Data Representation and Codes (SGDRC) in September 1993, with approval by the WGDM in
February 1994. These changes did not result in a new Edition to GRIB, but did change some
of the Tables, resulting a new Version number for them. This brings us now to Table
Version 2. The changes from Version 1 were mainly additions of new parameters or more
precise definition of existing ones.</p>
<p align="JUSTIFY">It is not anticipated that there will be any large-scale structural
changes to GRIB for at least four to five years, or more. The SGDRC is undertaking a
thorough review of the present and future requirements that GRIB is supposed to satisfy.
The plan is to design a major revision of GRIB capable of accommodating these requirements
and more, without "straining" the structure of the data representation form.
Some things are getting a little strained even now. See below. </p>
<p align="JUSTIFY">Note: the Edition number is placed in the same location, relative to
the start of the GRIB message, for all Editions. Thus, decoding programs can detect which
Edition was used to construct a particular GRIB message and behave accordingly. This is
useful for archives of messages encoded in earlier Editions or during transition periods.
Of course, this requires that data centers retain copies of older editions of the code,
and older versions of the parameter tables.</p>
<p align="JUSTIFY">Each GRIB record intended for either transmission or storage contains a
single parameter with values located at an array of grid points, or represented as a set
of spectral coefficients, for a single level (or layer), encoded as a continuous bit
stream. Logical divisions of the record are designated as "sections", each of
which provides control information and/or data. A GRIB record consists of six sections,
two of which are optional:</p>
<dir>
<dir>
<dir>
<p>(0) Indicator Section<br>
(1) Product Definition Section (PDS)<br>
(2) Grid Description Section (GDS) - optional<br>
(3) Bit Map Section (BMS) - optional<br>
(4) Binary Data Section (BDS)<br>
(5) '7777' (ASCII Characters)</p>
</dir>
</dir>
</dir>
<p align="JUSTIFY">Although the Grid Description Section is indicated as optional, it is
highly desirable that it be included in all messages. That way there will be no question
about just what is the "correct" geographical grid for a particular field.</p>
<p align="JUSTIFY">Most centers require bulletin headers to enable them to receive,
identify, and switch messages. The standard WMO abbreviated heading for GRIB is described
in Appendix A.</p>
<p align="JUSTIFY">In this documentation, certain symbols are used to clarify the contents
of octets (groups of eight consecutive binary bits). If unadorned letters are used, they
are symbolic and their meanings are described in the text; a decimal number is simply
printed as is; a character or string of characters is represented inside single quote
marks. International Alphabet No. 5, which is identical in its essential elements to the
U.S. National Standard 7-bit ASCII, is used for character representation in the GRIB code.
</p>
<p align="JUSTIFY">Octets are numbered consecutively from the start of each section; bits
within an octet are also numbered from left (the most significant bit) to right (the least
significant bit). Thus an octet with bit 8 set to the value 1 would have the integer value
1; bit 7 set to one would have a value of 2, etc.</p>
<p align="JUSTIFY">The numbering of Tables in the following text corresponds to the
description of GRIB in the WMO Manual on Codes. Some additional tables not found in the
WMO Manual are indicated by letters. These, generally, contain information unique to a
particular processing center.</p>
<p align="JUSTIFY">A caveat: The Official International Documentation for GRIB is the just
referenced Manual on Codes. This document is, in part, intended to be a guide to the use
of GRIB and may not include all of the features currently found in the Manual. The
features described here are intended to be a completely consistent sub-set of the full WMO
documentation; if there are any discrepancies the Manual on Codes is the final authority.</p>
<b>
<p align="JUSTIFY">DATA PACKING METHODS</p>
</b>
<p align="JUSTIFY">The code form represents numeric data as a series of binary digits
(bits). Such data representation is independent of any particular machine representation;
by convention data lengths are measured in octets. Data are coded as binary integers using
the minimum number of bits required for the desired precision. Numeric values, with units
as shown in Table 2, may first be scaled by a power of ten to achieve an appropriate
decimal precision, a reference value is subtracted from them to reduce redundancy and
eliminate negative values, and they may then be further scaled by a power of two to pack
them into a pre-selected word length. The two scaling operations are independent; which,
or both, are used in any given case depends upon choices made as to the method of packing.
See below.</p>
<p align="JUSTIFY">The representation of a single value is such that:</p>
<p align="CENTER">Y x 10<sup>D</sup> = R + (X x 2<sup>E</sup>)</p>
<dir>
<dir>
<dir>
<p>Where</p>
<p>Y = original or unpacked value; units as in Table 2;<br>
D = decimal scale factor, to achieve desired precision<br>
(sign bit, followed by a 15-bit integer);<br>
R = reference value (32 bits);<br>
X = internal value (No. of bits varies for each record);<br>
E = binary scale factor for variable bit word length packing<br>
(sign bit, followed by a 15-bit integer). </p>
</dir>
</dir>
</dir>
<p align="JUSTIFY">The reference value (R) is the minimum value of the (possibly)
decimally scaled data that is being encoded. </p>
<p align="JUSTIFY">R is placed in the Binary Data Section in four octets as a single
precision floating-point number:</p>
<p align="JUSTIFY">sAAAAAAA BBBBBBBB BBBBBBBB BBBBBBBB</p>
<dir>
<dir>
<dir>
<dir>
<dir>
<p>where s = sign bit, encoded as 0 =&gt; positive, 1 =&gt; negative<br>
A = 7-bit binary integer, the characteristic<br>
B = 24-bit binary integer, the mantissa.</p>
</dir>
</dir>
</dir>
</dir>
</dir>
<p>The appropriate formula to recover the value of R is:</p>
<p align="CENTER">R = (-1)<sup>s</sup> x 2<sup>(-24)</sup> x B x 16<sup>(A-64)</sup></p>
<p align="JUSTIFY">This formula is the standard IBM representation for a single precision
(real) floating point number. (Consideration is being given to using the IEEE floating
point representation in the future, in a later Edition of GRIB.)</p>
<p align="JUSTIFY">If second order (or "complex") packing is used (see the
description of that later on) the internal value, X, will be made up of two values, a
"local minimum value", Xi, and a "second order packed value", Xj.
There will be one Xj for each grid point and a variable number of Xi values. This will all
become clear later on when we get to the description of second-order packing.</p>
<p align="JUSTIFY">What follows is a description, slightly simplified, of the process that
one would go through to pack a (meteorological) field into a GRIB message, using
"simple packing". It includes some explanations of why certain steps are taken,
some of the consequences, and what choices have to be made. Some of the choices are
interrelated; the relationships should be clear when the explanation is done. The
additional features of complex or "second order" packing will be dealt with in a
later section.</p>
<p align="JUSTIFY">Give that a full field is available, the first step, if necessary, is
to convert the units of the parameter into those shown in Table 2, the SI standard units,
also known as the mks system. Some of the units may seem a little peculiar (kg/m<sup>2</sup>,
for example, for precipitation - 1 kg/m<sup>2 </sup>is equivalent to a water depth of 1
mm); others may seem inappropriate (Pa for pressure, for example, implies substantially
greater precision than is typical in meteorological usage; inverse seconds are not nearly
precise enough for divergence and vorticity) but they are all self consistent. The
precision of the parameters, as actually packed in a message, can be set to any desired
degree through the appropriate use of the power-of-10 ("D") scaling and the
power-of-2 ("E") scaling. Just how this comes about will be described
momentarily.</p>
<p align="JUSTIFY">At this point there is a choice to be made. If it is desired to use a
pre-selected bit word length for the packed variables, then just proceed on to the next
step. However, if a variable bit word length is to be used, where the word length is
adjusted to accommodate the data values, then it is necessary to undertake the
power-of-ten scaling. The D value should be selected such that, when the original data, in
the SI units of Table 2, is multiplied by 10<sup>D</sup>, the integer part of the result
will have enough precision to contain all the appropriate information of the variable.
Anticipating things a little bit, the (scaled) value will be rounded to an integer as a
part of the packing process; thus the "significant part" of the value of the
variable has to be moved to the left of the decimal point prior to the rounding.
Temperature might be scaled with D=1, thus changing the units to deci-degrees; pressure,
on the other hand, might be scaled with D=-2, thus actually reducing the precision to
hectoPascals (mb), a more reasonable meteorological precision; vorticity would be scaled
up by D=8, and so on.</p>
<p align="JUSTIFY">The second step in the packing operation is to scan through the field,
which may or may not have been "D-scaled" at this point, find the minimum value
of the parameter, and subtract that minimum - the reference value, R - from all the data
points, leaving a residual of non-negative numbers. This step has two benefits. The first
of these is convenience - making all the data points non-negative bypasses problems with
different computer hardware that represent negatives in various ways: 1's complement, 2's
complement, signed positive integers, whatever. The GRIB message is rendered just that
much more machine independent by being non-negative throughout.</p>
<p align="JUSTIFY">The second benefit is more consequential: it can result in a
substantial compression of the bulletin size without any loss of information content. If a
field has an appreciable bias away from zero, the residuals formed by the minimum removal
operation will all be much smaller numbers than otherwise. Thus they will need fewer bits
to contain them when they are, eventually, packed as integers.</p>
<p align="JUSTIFY">The third step is simply to scan through the field of residuals and
find the maximum value. </p>
<p align="JUSTIFY">At this point another choice must be made, similar to the one made
previously. This time, if a variable bit word length is to be used, then it is necessary
to calculate how many bits (per word or per data grid point) are going to be needed to
contain that largest data value, when the latter has been rounded to an integer. Recall
that at the previous decision point, the variables were power-of-ten ("D")
scaled such that a rounding operation will preserve all the significant part of the
information. Discovering how many bits are needed is a simple scan through a table of
powers of two, of course. The power-of-two-scaling is not employed and E is set equal to
0. Then go on to the fourth step.</p>
<p align="JUSTIFY">If, alternatively, it is desired to use a pre-selected bit word length
for the packed variables, the data must now be scaled, this time by a power of two (the
"E" scaling), sufficient to either reduce the maximum value down to just fit
into the available number of bits, or enlarge the value to just fit. This latter step
takes care of the problem of small numbers where the precision is all in the fractional
part of the number. How much precision is retained, for the eventual rounding, is a
function of the pre-selected bit word length and the "typical" range, or maximum
value with the minimum removed, of the particular variable. The choice of bit word length,
which is made ahead of time, must be made with full knowledge of the characteristics of
the particular variable that is to be packed and a prior assumption of how much precision
needs to be retained for the largest likely value.</p>
<p align="JUSTIFY">The fourth step is to round all the values to integers, now that they
have all been scaled to appropriate units, and pack them in the specified bit length
words. </p>
<p align="JUSTIFY">The last step is then to set up the various identification fields and
put the GRIB bulletin in proper form. We shall turn to this "proper form" in the
next section.</p>
<p align="JUSTIFY">We have ended up with two alternate ways to construct a GRIB messages:
a fixed bit word length method and a variable bit word length method. What are the
relative advantages or disadvantages, or at least the differences, of one with respect to
the other? </p>
<p align="JUSTIFY">Message length: the fixed word length bulletins are always the same
length, for a given parameter; the variable word length bulletins are, naturally,
variable. The variation is driven by the range of the value of the parameter over the
field (or the maximum value) which can change from day to day. Whether variations in
message length is a problem or not depends on the computer systems used to work with the
GRIB records.</p>
<p align="JUSTIFY">Precision: The variable word length bulletins have a fixed and
unchanging precision, determined by the "D" scaling. This assures that the same
information content is available day after day. It is straightforward to change the
precision in a familiar manner, that is, simply by orders of magnitude, just by altering
the D value. This comes at a cost, of course; increasing the precision by a power of 10
adds about 3.3 bits (average) to each data point in the message.</p>
<p align="JUSTIFY">The fixed word length bulletins show a variable precision which is case
by case data driven and is determined by the "E" (power-of-two) scaling that was
used to fit the numbers into the available space. This can happen even with the same data,
on the same date, but at adjacent grid areas. If one area shows a low variability and the
neighboring one a high variability such that a different power-of-two scaling is needed in
the two areas, then, unfortunately, the values on a common boundary will not be exactly
equal after they are unpacked. This can be disconcerting and a cause for confusion. It
will not happen if D-scaling (only) is employed. On the other hand, the variable precision
can be viewed as a strength: a data field with a low variability will be encoded at a
higher precision, thus preserving the character of the field; a high variability field
will be represented with less precision, but that is not a problem as the small, and
possibly lost, variations will not matter in the presence of the large ones. The precision
of the encoded field can be increased by adding bits to the fixed word length, but the
degree of change (a power of 2 for each bit) may not be as easy to deal with (or explain
to people) as the simple order of magnitude change afforded by the "D" scaling
method.</p>
<p align="JUSTIFY">No matter which packing method was employed, a proper GRIB decoding
program, that took account of the transmitted values of both "D" and
"E", would return the correct unpacked numbers, regardless of which packing
method was employed. It would be transparent to the user except for the questions of
precision outlined above.</p>
<b>
<p align="JUSTIFY"><a name="GRIB">GRIB CODE </a>FORM</p>
</b>
<p align="JUSTIFY">With the exception of the first four octets of the Indicator Section,
and the End Section, all octets contain binary values. All sections contain an even number
of octets; the variable length sections are padded with zero values as necessary. These
extra bits must be accounted for in finding one's way through the sections; their content
should be ignored.</p>
<b>
<p align="JUSTIFY"><a name="Section0">SECTION 0</a>: THE INDICATOR SECTION (IS)</p>
</b>
<p align="JUSTIFY">The indicator section serves to: identify the start of the record in a
human readable form, indicate the total length of the message, and indicate the Edition
number of GRIB used to construct or encode the message. The section is always eight octets
long. </p>
<table cellspacing="0" border="0" cellpadding="7" width="571">
<tbody><tr>
<td width="13%" valign="TOP"><p align="JUSTIFY">Octet no.</p></td>
<td width="87%" valign="TOP"><p align="JUSTIFY">IS Content</p></td>
</tr>
<tr>
<td width="13%" valign="TOP"><p align="JUSTIFY">1-4</p></td>
<td width="87%" valign="TOP"><p align="JUSTIFY">'GRIB' (Coded CCITT-ITA No. 5) (ASCII);</p></td>
</tr>
<tr>
<td width="13%" valign="TOP"><p align="JUSTIFY">5-7</p></td>
<td width="87%" valign="TOP"><p align="JUSTIFY">Total length, in octets, of GRIB
message(including Sections 0 &amp; 5);</p></td>
</tr>
<tr>
<td width="13%" valign="TOP"><p align="JUSTIFY">8</p></td>
<td width="87%" valign="TOP"><p align="JUSTIFY">Edition number - currently 1</p></td>
</tr>
</tbody></table>
<b>
<p align="JUSTIFY"><a name="Section1">SECTION 1</a>: THE PRODUCT DEFINITION SECTION (PDS).</p>
</b>
<p align="JUSTIFY">The PDS contains indicators for the Parameter table Version, the
originating center, the numerical model (or "generating process") that created
the data, the geographical area covered by the data, the parameter itself, the values for
the appropriate vertical level or layer where the data reside, the decimal scale factor,
and date/time information. The PDS is normally 28 octets long but it may be longer if an
originating center chooses to make it so. Users of GRIB messages are strongly urged to use
the length-of-section portion of the PDS to determine where the next section begins. Never
assume a fixed octet length in this, or any other, section.</p>
<table cellspacing="0" border="0" cellpadding="7" width="608">
<tbody><tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">Octet no.</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">PDS Content</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">1 - 3</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Length in octets of the
Product Definition Section</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">4</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Parameter Table Version
number.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Currently Version 2 for
international exchange.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Parameter table version
numbers 128-254 are reserved for local use.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">5</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Identification of center
(Table 0 - Part 1)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">6</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Generating process ID number </p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">(allocated by the originating
center; See Table A)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">7</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Grid Identification
(geographical location and area; See Table B)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">8</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Flag specifying the presence
or absence of a GDS or a BMS</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">(See Table 1)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">9</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Indicator of parameter and
units (Table 2)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">10</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Indicator of type of level or
layer (See Tables 3 &amp; 3a)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">11-12</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Height, pressure, etc. of the
level or layer (See Table 3)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP">13<br>
14<br>
<br>
15<br>
<br>
16<br>
17</td>
<td width="29%" valign="TOP">Year of century<br>
Month of year <br>
<br>
Day of month <br>
<br>
Hour of day <br>
Minute of hour </td>
<td width="57%" valign="TOP">\ Initial (or Reference)<br>
| time of forecast - UTC<br>
| <br>
| &gt; or<br>
|<br>
| for averaging or <br>
| accumulation of<br>
/ analyses</td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">18</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Forecast time unit (see Table
4)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">19</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">P1 - Period of time (Number of
time units)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">(0 for analysis or initialized
analysis.)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Units of time given by content
of octet 18.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP" height="68"><p align="JUSTIFY">20</p></td>
<td width="86%" valign="TOP" colspan="2" height="68"><p align="JUSTIFY">P2 - Period of
time (Number of time units) or</p>
<p align="JUSTIFY">Time interval between successive analyses, successive initialized
analyses, or forecasts, undergoing averaging or accumulation. Units given by octet 18.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">21</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Time range indicator (See
Table 5)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">22-23</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Number included in average,
when octet 21 (Table 5) indicates an average or accumulation; otherwise set to zero.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">24</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Number Missing from averages
or accumulations.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">25</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Century of Initial (Reference)
time (=20 until Jan. 1, 2001)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">26</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Identification of sub-center
(Table 0 - Part 2)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">27-28</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">The decimal scale factor D. A
negative value is indicated by setting the high order bit (bit No. 1) in octet 27 to 1
(on).</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">29-40</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Reserved (need not be present)</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">41-...</p></td>
<td width="86%" valign="TOP" colspan="2"><p align="JUSTIFY">Reserved for originating
center use.</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp; Note: Octet 8 may indicate the presence of the Grid Description
Section (GDS) even though octet 7 specifies a predefined grid. In this case the GDS must
describe that grid -this device serves as a mechanism for transmitting new
"predefined" grids to users prior to their formal publication in this or the
official WMO documentation. It is, however, the desired practice to always include the GDS
in GRIB bulletins.</p>
<p align="JUSTIFY">Another Note: The use of octet 26 to indicate a "sub-center"
is not (yet) an officially sanctioned WMO practice. The Manual indicates the octet is to
be "reserved", and set to 0. The use arises out of an upcoming change in the
Manual in which the "originating centers" for both GRIB and BUFR (FM 94) will
reference a single common table. The difficulty is that BUFR has two octets available for
an originating center number while GRIB has only one. The compromise solution is to allow
the use of octet 26 as the "second" octet for GRIB, but only in a national
context. The WMO will coordinate the assignment of the numbers for octet 5 for national
and international centers (for both GRIB and BUFR), while each national center will then
be free to assign sub-center numbers at will to be placed in the octet 26. A zero value in
octet 26 will serve as the default indicating that there is no sub-center associated with
a particular center. Table 0, in this document, shows, in Part 1, a selection of the WMO
recognized originating centers (those that are currently active) as would be found in
octet 5.</p>
<p align="JUSTIFY">&nbsp;</p>
<b>
<p align="CENTER">TABLES FOR THE PDS</p>
</b>
<p align="CENTER">TABLE 0 - Part 1</p>
<p align="CENTER">NATIONAL/INTERNATIONAL</p>
<p align="CENTER">ORIGINATING CENTERS</p>
<p align="CENTER">(Assigned By The WMO)</p>
<p align="CENTER">(PDS Octet 5)</p>
<table cellspacing="0" border="0" cellpadding="7" width="505">
<tbody><tr>
<td width="14%" valign="TOP">Value</td>
<td width="86%" valign="TOP">Contents</td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP"></td>
</tr>
<tr>
<td width="14%" valign="TOP">07</td>
<td width="86%" valign="TOP">US Weather Service - National Met. Center</td>
</tr>
<tr>
<td width="14%" valign="TOP">08</td>
<td width="86%" valign="TOP">US Weather Service - NWS Telecomms Gateway</td>
</tr>
<tr>
<td width="14%" valign="TOP">09</td>
<td width="86%" valign="TOP">US Weather Service - Field Stations</td>
</tr>
<tr>
<td width="14%" valign="TOP">34</td>
<td width="86%" valign="TOP">Japanese Meteorological Agency - Tokyo</td>
</tr>
<tr>
<td width="14%" valign="TOP">52</td>
<td width="86%" valign="TOP">National Hurricane Center, Miami</td>
</tr>
<tr>
<td width="14%" valign="TOP">54</td>
<td width="86%" valign="TOP">Canadian Meteorological Service - Montreal</td>
</tr>
<tr>
<td width="14%" valign="TOP">57</td>
<td width="86%" valign="TOP">U.S. Air Force - Global Weather Center</td>
</tr>
<tr>
<td width="14%" valign="TOP">58</td>
<td width="86%" valign="TOP">US Navy - Fleet Numerical Oceanography Center</td>
</tr>
<tr>
<td width="14%" valign="TOP">59</td>
<td width="86%" valign="TOP">NOAA Forecast Systems Lab, Boulder CO</td>
</tr>
<tr>
<td width="14%" valign="TOP">74</td>
<td width="86%" valign="TOP">U.K. Met Office - Bracknell</td>
</tr>
<tr>
<td width="14%" valign="TOP">85</td>
<td width="86%" valign="TOP">French Weather Service - Toulouse</td>
</tr>
<tr>
<td width="14%" valign="TOP">97</td>
<td width="86%" valign="TOP">European Space Agency (ESA)</td>
</tr>
<tr>
<td width="14%" valign="TOP">98</td>
<td width="86%" valign="TOP">European Center for Medium-Range Weather Forecasts - Reading</td>
</tr>
<tr>
<td width="14%" valign="TOP">99</td>
<td width="86%" valign="TOP">DeBilt, Netherlands</td>
</tr>
</tbody></table>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE 0 - Part 2</p>
<p align="CENTER">NATIONAL SUB-CENTERS</p>
<p align="CENTER">(Assigned By The Nation)</p>
<p align="CENTER">(PDS Octet 26)</p>
<p align="CENTER">National sub-centers, if any, would be found here.</p>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE A.Generating Process or Model</p>
<p align="CENTER">(PDS Octet 6)</p>
<dir>
<dir>
<p>VALUE MODEL</p>
</dir>
</dir>
<p>These values and model ("generating process") numbers would be supplied by
the individual centers.</p>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE B. GRID IDENTIFICATION</p>
<p align="CENTER">(PDS Octet 7)</p>
<p align="CENTER">Various National Grids would be identified here by the Center
International exchange grids are included here. </p>
<p align="CENTER">&nbsp;</p>
<table cellspacing="0" border="0" cellpadding="7" width="619">
<tbody><tr>
<td width="15%" valign="TOP">VALUE</td>
<td width="85%" valign="TOP">GRID</td>
</tr>
<tr>
<td width="15%" valign="TOP">21-26</td>
<td width="85%" valign="TOP">International Exchange grids - see below</td>
</tr>
<tr>
<td width="15%" valign="TOP">37-44</td>
<td width="85%" valign="TOP">Eight lat-long 1.25 x 1.25 "thinned" grids,
covering the globe by octants of 3447 points. Full GRIB specifications below. For WAFC,
ICAO, and International exchange.</td>
</tr>
<tr>
<td width="15%" valign="TOP">61-64</td>
<td width="85%" valign="TOP">International Exchange grids - see below.</td>
</tr>
<tr>
<td width="15%" valign="TOP">255</td>
<td width="85%" valign="TOP">(non-defined grid - specified in the GDS)</td>
</tr>
</tbody></table>
<p>&nbsp;</p>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">INTERNATIONAL EXCHANGE GRIDS</p>
<table cellspacing="0" border="0" cellpadding="7" width="691">
<tbody><tr>
<td width="12%" valign="TOP"><p align="CENTER">VALUE</p></td>
<td width="16%" valign="TOP"><p align="CENTER">RESOLUTION</p></td>
<td width="22%" valign="TOP"><p align="CENTER">AREA</p></td>
<td width="36%" valign="TOP" colspan="2"><p align="CENTER">GRID</p></td>
<td width="15%" valign="TOP"><p align="CENTER">GRID</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="16%" valign="TOP"><p align="CENTER">(degrees)</p></td>
<td width="22%" valign="TOP"><p align="CENTER">COVERAGE</p></td>
<td width="36%" valign="TOP" colspan="2"><p align="CENTER">SHAPE</p></td>
<td width="15%" valign="TOP"><p align="CENTER">POINTS</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="16%" valign="TOP"><p align="CENTER">lon x lat</p></td>
<td width="22%" valign="TOP"><p align="CENTER">(degrees)</p></td>
<td width="18%" valign="TOP"><p align="CENTER">cols</p></td>
<td width="18%" valign="TOP"><p align="CENTER">rows</p></td>
<td width="15%" valign="TOP"></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">21</p></td>
<td width="16%" valign="TOP"><p align="CENTER">5.0 x 2.5</p></td>
<td width="22%" valign="TOP"><p align="CENTER">0-180E, 0-90N</p></td>
<td width="18%" valign="TOP"><p align="CENTER">37</p></td>
<td width="18%" valign="TOP"><p align="CENTER">36 + pole</p></td>
<td width="15%" valign="TOP"><p align="CENTER">1333</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">22</p></td>
<td width="16%" valign="TOP"><p align="CENTER">5.0 x 2.5</p></td>
<td width="22%" valign="TOP"><p align="CENTER">180W-0, 0-90N</p></td>
<td width="18%" valign="TOP"><p align="CENTER">37</p></td>
<td width="18%" valign="TOP"><p align="CENTER">36 + pole</p></td>
<td width="15%" valign="TOP"><p align="CENTER">1333</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">23</p></td>
<td width="16%" valign="TOP"><p align="CENTER">5.0 x 2.5</p></td>
<td width="22%" valign="TOP"><p align="CENTER">0-180E, 90S-0</p></td>
<td width="18%" valign="TOP"><p align="CENTER">pole + 37</p></td>
<td width="18%" valign="TOP"><p align="CENTER">36</p></td>
<td width="15%" valign="TOP"><p align="CENTER">1333</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">24</p></td>
<td width="16%" valign="TOP"><p align="CENTER">5.0 x 2.5</p></td>
<td width="22%" valign="TOP"><p align="CENTER">180W-0, 90S-0</p></td>
<td width="18%" valign="TOP"><p align="CENTER">pole + 37</p></td>
<td width="18%" valign="TOP"><p align="CENTER">36</p></td>
<td width="15%" valign="TOP"><p align="CENTER">1333</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">25</p></td>
<td width="16%" valign="TOP"><p align="CENTER">5.0 x 5.0</p></td>
<td width="22%" valign="TOP"><p align="CENTER">0-355E, 0-90N</p></td>
<td width="18%" valign="TOP"><p align="CENTER">72</p></td>
<td width="18%" valign="TOP"><p align="CENTER">18 + pole</p></td>
<td width="15%" valign="TOP"><p align="CENTER">1297</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">26</p></td>
<td width="16%" valign="TOP"><p align="CENTER">5.0 x 5.0</p></td>
<td width="22%" valign="TOP"><p align="CENTER">0-355E, 90S-0</p></td>
<td width="18%" valign="TOP"><p align="CENTER">pole + 72</p></td>
<td width="18%" valign="TOP"><p align="CENTER">18</p></td>
<td width="15%" valign="TOP"><p align="CENTER">1297</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">61</p></td>
<td width="16%" valign="TOP"><p align="CENTER">2.0 x 2.0</p></td>
<td width="22%" valign="TOP"><p align="CENTER">0-180E, 0-90N</p></td>
<td width="18%" valign="TOP"><p align="CENTER">91</p></td>
<td width="18%" valign="TOP"><p align="CENTER">45 + pole</p></td>
<td width="15%" valign="TOP"><p align="CENTER">4096</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">62</p></td>
<td width="16%" valign="TOP"><p align="CENTER">2.0 x 2.0</p></td>
<td width="22%" valign="TOP"><p align="CENTER">180W-0, 0-90N</p></td>
<td width="18%" valign="TOP"><p align="CENTER">91</p></td>
<td width="18%" valign="TOP"><p align="CENTER">45 + pole</p></td>
<td width="15%" valign="TOP"><p align="CENTER">4096</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">63</p></td>
<td width="16%" valign="TOP"><p align="CENTER">2.0 x 2.0</p></td>
<td width="22%" valign="TOP"><p align="CENTER">0-180E, 90S-0</p></td>
<td width="18%" valign="TOP"><p align="CENTER">pole + 91</p></td>
<td width="18%" valign="TOP"><p align="CENTER">45</p></td>
<td width="15%" valign="TOP"><p align="CENTER">4096</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">64</p></td>
<td width="16%" valign="TOP"><p align="CENTER">2.0 x 2.0</p></td>
<td width="22%" valign="TOP"><p align="CENTER">180W-0, 90S-0</p></td>
<td width="18%" valign="TOP"><p align="CENTER">pole + 91</p></td>
<td width="18%" valign="TOP"><p align="CENTER">45</p></td>
<td width="15%" valign="TOP"><p align="CENTER">4096</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="CENTER">255</p></td>
<td width="89%" valign="TOP" colspan="5">(non-standard grid - defined in the GDS)</td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp;NOTES ON INTERNATIONAL EXCHANGE/FOS GRIDS:</p>
<p align="JUSTIFY">&nbsp;(i) The grid points are laid out in a linear array such that the
longitude index (the columns) is the most rapidly varying. For the northern hemisphere
grids the first point in the record is at the intersection of the western-most meridian
and southern-most circle of latitude; the last point is the single polar value (see note
iii, below). For the southern hemisphere grids the first point in the record is the single
polar value (see note iii, below); the last point is at the intersection of the
eastern-most meridian and northern-most circle of latitude. For those familiar with
FORTRAN subscripting conventions, longitude is the first subscript, latitude the second.</p>
<p align="JUSTIFY">(ii) In grids 21 through 26, and 61 through 64, the values on the
shared boundaries are included in each area.</p>
<p align="JUSTIFY">(iii) The datum for the pole point is given only once in each grid. The
user must expand, if desired, the single pole point value to all the pole
"points" at the pole row of a latitude-longitude grid. Scalar quantity values
are the same for all pole points on a the grid. Wind components at the poles are given by
the formulae:</p>
<p align="CENTER">u = -speed x sin(dd) &amp; v = -speed x cos(dd)</p>
<p>&nbsp;where dd is the direction of the wind as reported according to the specification
of wind direction at the poles (refer to WMO Manual on Codes &lt;1&gt;, code table 878).</p>
<p>&nbsp; The WMO convention can be given this operational definition: At the North Pole,
face into the wind and report the value of the west longitude meridian along which the
wind is coming at you; at the South Pole do likewise but report the east longitude
meridian value. This is equivalent to placing the origin of a right-handed Cartesian
coordinate system on the North Pole with the y-axis pointing to the prime (0 degree)
meridian and the x-axis pointing to the 90 degrees west meridian, and then resolving any
vector wind at the pole point into components along those axes. At the South Pole the
coordinate axes are oriented such that the y-axis points toward 180 degrees west. Those
components are the u- and v-values given as the single pair of pole point winds in the
GRIB format.</p>
<p align="JUSTIFY">In terms of a longitude/latitude grid these are the wind components for
the pole point at the 180 degree meridian. For example, on a 2.5 x 2.5 degree northern
hemisphere grid (145 x 37 points), with the abscissa along the equator and the ordinate
along the prime meridian, the transmitted north pole wind components are those that belong
at the grid point (73,37). The wind components at the other grid points along the pole row
may be obtained through suitable rotation of the coordinate system. All the components at
the pole row are, of course, simply representations of the same vector wind viewed from
differing (rotated) coordinate systems. In the southern hemisphere the analogous situation
holds; the single set of transmitted pole point wind components belong at the grid point
(73,1). </p>
<p align="CENTER">WAFS/ICAO/INTERNATIONAL EXCHANGE/FOS GRIDS</p>
<p align="CENTER">(Grids 37 - 44)</p>
<p align="CENTER"><img src="./GuidetoGRIB_files/Guide-GRIB-1.gif" width="525" height="244">1</p>
<p align="CENTER">Global Coverage of Grids</p>
<p align="CENTER">Octants of the Globe</p>
<p align="JUSTIFY">In the figure the boxes indicate the location of the octants of the
globe, the numbers are the corresponding grid identification numbers (PDS Octet 7), and
the letters are the grid identification used in the WMO heading (see Appendix A). The left
and right meridional columns of each octant/grid are shared with the neighbors.</p>
<p align="JUSTIFY">The basic grid point separation is 1.25x1.25 deg. on a
latitude/longitude array, but the grid is "thinned" by reducing the number of
points in each row as one goes northward (or southward) away from the equator. In GRIB
terms, this is referred to as a "quasi-regular" grid.</p>
<p align="JUSTIFY">The latitudinal increment is always 1.25 deg.; this results in 73 rows
where the pole is included as a "row", not a single grid point. </p>
<p align="JUSTIFY">The longitudinal spacing at the equator is also 1.25 deg.; thus there
will be 73 grid points at the equator in each octant.</p>
<p align="JUSTIFY">The number of points on each latitudinal row, other than the equator,
is given by (using FORTRAN notation):</p>
<p align="CENTER">NPOINTS = IFIX(2.0 + (90.0/1.25) x COS(LATITUDE))</p>
<p>&nbsp;Thus at the pole there will be two grid points, one each at the meridians that
delineate the edges of the octant. The formula was worked out so that there is
(approximately) equal geographic separation between the grid points uniformly across the
globe.</p>
<p align="JUSTIFY">&nbsp; Because of variations in precision and round off error in
different computers, the value of NPOINTS may vary by 1 at "critical" latitudes
when calculated on various hardware platforms. Here is a table of the exact values of
NPOINTS as a function of latitude as used in the internationally exchanged grids. These
numbers will, of course, be found in the Grid Description Section of each GRIB bulletin.</p>
<p align="JUSTIFY">&nbsp;</p>
<table cellspacing="0" border="0" cellpadding="7" width="552">
<tbody><tr>
<td width="31%" valign="TOP" height="64">Latitude Range<br>
inclusive<br>
(north or south)</td>
<td width="23%" valign="TOP" height="64">NPOINTS&nbsp;</td>
<td width="29%" valign="TOP" height="64">Latitude Range<br>
Inclusive<br>
(north or south)</td>
<td width="17%" valign="TOP" height="64">NPOINTS&nbsp;</td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">0.00 - 8.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">73</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">55.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">43</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">10.00 - 12.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">72</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">56.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">42</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">13.75 - 16.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">71</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">57.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">40</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">17.50 - 18.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">70</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">58.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">39</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">20.00 - 21.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">69</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">60.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">38</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">22.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">68</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">61.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">36</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">23.75 - 25.00</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">65</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">62.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">35</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">30.00</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">64</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">63.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">33</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">31.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">63</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">65.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">32</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">32.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">62</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">66.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">30</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">33.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">61</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">67.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">29</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">35.00 - 36.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">60</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">68.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">28</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">37.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">59</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">70.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">26</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">38.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">58</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">71.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">25</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">40.00</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">57</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">72.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">23</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">41.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">56</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">73.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">22</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">42.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">55</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">75.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">20</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">43.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">54</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">76.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">19</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">45.00</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">52</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">77.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">17</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">46.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">51</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">78.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">16</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">47.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">50</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">80.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">14</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">48.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">49</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">81.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">12</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">50.00</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">48</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">82.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">11</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">51.25</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">47</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">83.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">9</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">52.50</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">45</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">85.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">8</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"><p align="JUSTIFY">53.75</p></td>
<td width="23%" valign="TOP"><p align="JUSTIFY">44</p></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">86.25</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">6</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"></td>
<td width="23%" valign="TOP"></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">87.50</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">5</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"></td>
<td width="23%" valign="TOP"></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">88.75</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">3</p></td>
</tr>
<tr>
<td width="31%" valign="TOP"></td>
<td width="23%" valign="TOP"></td>
<td width="29%" valign="TOP"><p align="JUSTIFY">90.00</p></td>
<td width="17%" valign="TOP"><p align="JUSTIFY">2</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp; When all this is put together the result is that there are 3447
points of data actually transmitted in any individual GRIB bulletin containing one octant
of the globe.</p>
<p align="JUSTIFY">In the GRIB bulletins all of this information will be included in the
Grid Description Section (GDS); the GDS must be included in order to describe the thinned
or "quasi-regular" grid structure. See Section 2 and Table C for the general
description of the GDS; what follows are the specific values of the variables in the GDS
that describe these eight grids.</p>
<p align="CENTER">GDS Contents</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="472">
<tbody><tr>
<td width="30%" valign="TOP">Octets</td>
<td width="70%" valign="TOP">Value or variable</td>
</tr>
<tr>
<td width="30%" valign="TOP"></td>
<td width="70%" valign="TOP"></td>
</tr>
<tr>
<td width="30%" valign="TOP">1-3</td>
<td width="70%" valign="TOP">178 (length of GDS)</td>
</tr>
<tr>
<td width="30%" valign="TOP">4</td>
<td width="70%" valign="TOP">0 (or 255, either indicating no PV)</td>
</tr>
<tr>
<td width="30%" valign="TOP">5</td>
<td width="70%" valign="TOP">33 (pointer to start of PL list)</td>
</tr>
<tr>
<td width="30%" valign="TOP">6</td>
<td width="70%" valign="TOP">0</td>
</tr>
<tr>
<td width="30%" valign="TOP">7-32</td>
<td width="70%" valign="TOP">Grid description - see below</td>
</tr>
<tr>
<td width="30%" valign="TOP">33-178</td>
<td width="70%" valign="TOP">Number of points in each of 73 rows</td>
</tr>
<tr>
<td width="30%" valign="TOP"></td>
<td width="70%" valign="TOP">(2 octets per point)</td>
</tr>
<tr>
<td width="30%" valign="TOP"></td>
<td width="70%" valign="TOP"></td>
</tr>
</tbody></table>
</center></div>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">Details of Octets 7-32 - Grid Description</p>
<div align="center"><center>
<table border="1" cellspacing="2" cellpadding="7" width="636">
<tbody><tr>
<td width="23%" valign="TOP" colspan="5">&nbsp;</td>
<td width="77%" valign="TOP" colspan="15">&nbsp;</td>
</tr>
<tr>
<td width="23%" valign="TOP" colspan="5">Octets</td>
<td width="77%" valign="TOP" colspan="15">Variable &amp; Value</td>
</tr>
<tr>
<td width="23%" valign="TOP" colspan="5">&nbsp;</td>
<td width="77%" valign="TOP" colspan="15">&nbsp;</td>
</tr>
<tr>
<td width="23%" valign="TOP" colspan="5">7-8</td>
<td width="77%" valign="TOP" colspan="15">Ni = all bits set to 1 (missing)</td>
</tr>
<tr>
<td width="23%" valign="TOP" colspan="5">9-10</td>
<td width="77%" valign="TOP" colspan="15">Nj = 73</td>
</tr>
<tr>
<td width="10%" valign="TOP" colspan="2">&nbsp;&nbsp;</td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">GRID:</p></td>
<td width="10%" valign="TOP" colspan="3"><p align="CENTER">37</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">38</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">39</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">40</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">41</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">42</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">43</p></td>
<td width="10%" valign="TOP"><p align="CENTER">44</p></td>
</tr>
<tr>
<td width="10%" valign="TOP" colspan="2">11-13</td>
<td width="10%" valign="TOP" colspan="2">La1 =</td>
<td width="10%" valign="TOP" colspan="3"><p align="CENTER">0</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">90S</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">90S</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">90S</p></td>
<td width="10%" valign="TOP"><p align="CENTER">90S</p></td>
</tr>
<tr>
<td width="10%" valign="TOP" colspan="2">14-16</td>
<td width="10%" valign="TOP" colspan="2">Lo1 =</td>
<td width="10%" valign="TOP" colspan="3"><p align="CENTER">330</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">60</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">150</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">240</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">330</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">60</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">150</p></td>
<td width="10%" valign="TOP"><p align="CENTER">240</p></td>
</tr>
<tr>
<td width="10%" valign="TOP" colspan="2">17</td>
<td width="10%" valign="TOP" colspan="2">&nbsp;</td>
<td width="80%" valign="TOP" colspan="16">Resolution &amp; Component Flag = [10000000]
(binary)</td>
</tr>
<tr>
<td width="10%" valign="TOP">&nbsp;&nbsp;&nbsp;</td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">GRID:</p></td>
<td width="10%" valign="TOP" colspan="3"><p align="CENTER">37</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">38</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">39</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">40</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">41</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">42</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">43</p></td>
<td width="11%" valign="TOP" colspan="2"><p align="CENTER">44</p></td>
</tr>
<tr>
<td width="10%" valign="TOP">18-20</td>
<td width="10%" valign="TOP" colspan="2">La2 =</td>
<td width="10%" valign="TOP" colspan="3"><p align="CENTER">90N</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">90N</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">90N</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">90N</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
<td width="11%" valign="TOP" colspan="2"><p align="CENTER">0</p></td>
</tr>
<tr>
<td width="10%" valign="TOP">21-23</td>
<td width="10%" valign="TOP" colspan="2">Lo2 =</td>
<td width="10%" valign="TOP" colspan="3"><p align="CENTER">60</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">150</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">240</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">330</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">60</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">150</p></td>
<td width="10%" valign="TOP" colspan="2"><p align="CENTER">240</p></td>
<td width="11%" valign="TOP" colspan="2"><p align="CENTER">330</p></td>
</tr>
<tr>
<td width="10%" valign="TOP">24-25</td>
<td width="10%" valign="TOP" colspan="2">&nbsp;</td>
<td width="80%" valign="TOP" colspan="17">Di = all bits set to 1 (missing)</td>
</tr>
<tr>
<td width="10%" valign="TOP">26-27</td>
<td width="10%" valign="TOP" colspan="2">&nbsp;</td>
<td width="80%" valign="TOP" colspan="17">Dj = 1.25 deg</td>
</tr>
<tr>
<td width="10%" valign="TOP">28</td>
<td width="10%" valign="TOP" colspan="2">&nbsp;</td>
<td width="80%" valign="TOP" colspan="17">Scan Mode = [01000000] (binary)</td>
</tr>
<tr>
<td width="10%" valign="TOP">29-32</td>
<td width="10%" valign="TOP" colspan="2">&nbsp;</td>
<td width="80%" valign="TOP" colspan="17">Set to 0 (unused)</td>
</tr>
</tbody></table>
</center></div>
<p>&nbsp; Note that the scanning direction is from the bottom (south edge) to the top of
the octant grids, regardless of the hemisphere. Thus in the northern hemisphere the first
73 data points (in the BDS) will be the equatorial values and the last two will be the
polar values. The PL counts in the GDS octets 33-178 will, of course, indicate contain
these numbers. </p>
<p align="JUSTIFY">In the southern hemisphere, the first two data points will be the south
pole values, and the last 73 points will be the equatorial values. Octets 33-34 in the GDS
will contain "2", octets 35-36 will contain a "3", and so on to octets
177-178 which will contain "73".</p>
<p align="CENTER">TABLE 1. FLAG FOR GDS OR BMS</p>
<p align="CENTER">(PDS Octet 8)</p>
<p align="CENTER">The bit flag indicates the omission or inclusion of the Grid Description
and/or Bit Map Sections.</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="428">
<tbody><tr>
<td width="23%" valign="TOP">BIT</td>
<td width="23%" valign="TOP">VALUE</td>
<td width="54%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="23%" valign="TOP">1</td>
<td width="23%" valign="TOP">0</td>
<td width="54%" valign="TOP">GDS Omitted</td>
</tr>
<tr>
<td width="23%" valign="TOP"></td>
<td width="23%" valign="TOP">1</td>
<td width="54%" valign="TOP">GDS Included</td>
</tr>
<tr>
<td width="23%" valign="TOP">2</td>
<td width="23%" valign="TOP">0</td>
<td width="54%" valign="TOP">BMS Omitted</td>
</tr>
<tr>
<td width="23%" valign="TOP"></td>
<td width="23%" valign="TOP">1</td>
<td width="54%" valign="TOP">BMS Included</td>
</tr>
<tr>
<td width="23%" valign="TOP">3-8</td>
<td width="23%" valign="TOP">0</td>
<td width="54%" valign="TOP">reserved</td>
</tr>
</tbody></table>
</center></div>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE 2. PARAMETERS &amp; UNITS</p>
<p align="CENTER">Version 2</p>
<p align="CENTER">(PDS Octet 9)</p>
<table cellspacing="0" border="0" cellpadding="7" width="601">
<tbody><tr>
<td width="26%" valign="TOP">VALUE</td>
<td width="58%" valign="TOP">PARAMETER</td>
<td width="16%" valign="TOP">UNITS</td>
</tr>
<tr>
<td width="26%" valign="TOP">000</td>
<td width="58%" valign="TOP">Reserved</td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">001</td>
<td width="58%" valign="TOP">Pressure</td>
<td width="16%" valign="TOP">Pa</td>
</tr>
<tr>
<td width="26%" valign="TOP">002</td>
<td width="58%" valign="TOP">Pressure reduced to MSL</td>
<td width="16%" valign="TOP">Pa</td>
</tr>
<tr>
<td width="26%" valign="TOP">003</td>
<td width="58%" valign="TOP">Pressure tendency </td>
<td width="16%" valign="TOP">Pa/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">004</td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">005</td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">006</td>
<td width="58%" valign="TOP">Geopotential</td>
<td width="16%" valign="TOP">m<sup>2</sup>/s<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">007</td>
<td width="58%" valign="TOP">Geopotential height </td>
<td width="16%" valign="TOP">Gpm</td>
</tr>
<tr>
<td width="26%" valign="TOP">008</td>
<td width="58%" valign="TOP">Geometric height</td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">009</td>
<td width="58%" valign="TOP">Standard deviation of height</td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">010</td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">011</td>
<td width="58%" valign="TOP">Temperature </td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">012</td>
<td width="58%" valign="TOP">Virtual temperature </td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">013</td>
<td width="58%" valign="TOP">Potential temperature</td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">014</td>
<td width="58%" valign="TOP">Pseudo-adiabatic potential temperature </td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">015</td>
<td width="58%" valign="TOP">Maximum temperature</td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">016</td>
<td width="58%" valign="TOP">Minimum temperature </td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">017</td>
<td width="58%" valign="TOP">Dew point temperature</td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">018</td>
<td width="58%" valign="TOP">Dew point depression (or deficit)</td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">019</td>
<td width="58%" valign="TOP">Lapse rate</td>
<td width="16%" valign="TOP">K/m</td>
</tr>
<tr>
<td width="26%" valign="TOP">020</td>
<td width="58%" valign="TOP">Visibility </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">021</td>
<td width="58%" valign="TOP">Radar Spectra (1) </td>
<td width="16%" valign="TOP">-</td>
</tr>
<tr>
<td width="26%" valign="TOP">022</td>
<td width="58%" valign="TOP">Radar Spectra (2) </td>
<td width="16%" valign="TOP">-</td>
</tr>
<tr>
<td width="26%" valign="TOP">023</td>
<td width="58%" valign="TOP">Radar Spectra (3) </td>
<td width="16%" valign="TOP">-</td>
</tr>
<tr>
<td width="26%" valign="TOP">024 </td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">025</td>
<td width="58%" valign="TOP">Temperature anomaly </td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">026</td>
<td width="58%" valign="TOP">Pressure anomaly </td>
<td width="16%" valign="TOP">Pa</td>
</tr>
<tr>
<td width="26%" valign="TOP">027</td>
<td width="58%" valign="TOP">Geopotential height anomaly </td>
<td width="16%" valign="TOP">Gpm</td>
</tr>
<tr>
<td width="26%" valign="TOP">028</td>
<td width="58%" valign="TOP">Wave Spectra (1)</td>
<td width="16%" valign="TOP">-</td>
</tr>
<tr>
<td width="26%" valign="TOP">029</td>
<td width="58%" valign="TOP">Wave Spectra (2)</td>
<td width="16%" valign="TOP">-</td>
</tr>
<tr>
<td width="26%" valign="TOP">030</td>
<td width="58%" valign="TOP">Wave Spectra (3)</td>
<td width="16%" valign="TOP">-</td>
</tr>
<tr>
<td width="26%" valign="TOP">031</td>
<td width="58%" valign="TOP">Wind direction</td>
<td width="16%" valign="TOP">Deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">032</td>
<td width="58%" valign="TOP">Wind speed </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">033</td>
<td width="58%" valign="TOP">u-component of wind</td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">034</td>
<td width="58%" valign="TOP">v-component of wind </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">035</td>
<td width="58%" valign="TOP">Stream function </td>
<td width="16%" valign="TOP">m<sup>2</sup>/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">036</td>
<td width="58%" valign="TOP">Velocity potential</td>
<td width="16%" valign="TOP">m<sup>2</sup>/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">037</td>
<td width="58%" valign="TOP">Montgomery stream function </td>
<td width="16%" valign="TOP">m<sup>2</sup>/s<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">038</td>
<td width="58%" valign="TOP">Sigma coord. vertical velocity </td>
<td width="16%" valign="TOP">s /s</td>
</tr>
<tr>
<td width="26%" valign="TOP">039</td>
<td width="58%" valign="TOP">Pressure Vertical velocity </td>
<td width="16%" valign="TOP">Pa/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">040</td>
<td width="58%" valign="TOP">Geometric Vertical velocity </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">041</td>
<td width="58%" valign="TOP">Absolute vorticity </td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">042</td>
<td width="58%" valign="TOP">Absolute divergence </td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">043</td>
<td width="58%" valign="TOP">Relative vorticity </td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">044</td>
<td width="58%" valign="TOP">Relative divergence </td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">045</td>
<td width="58%" valign="TOP">Vertical u-component shear</td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">046</td>
<td width="58%" valign="TOP">Vertical v-component shear</td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">047</td>
<td width="58%" valign="TOP">Direction of current </td>
<td width="16%" valign="TOP">Deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">048</td>
<td width="58%" valign="TOP">Speed of current</td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">049</td>
<td width="58%" valign="TOP">u-component of current</td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">050</td>
<td width="58%" valign="TOP">v-component of current</td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">051</td>
<td width="58%" valign="TOP">Specific humidity </td>
<td width="16%" valign="TOP">kg/kg</td>
</tr>
<tr>
<td width="26%" valign="TOP">052</td>
<td width="58%" valign="TOP">Relative humidity</td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">053</td>
<td width="58%" valign="TOP">Humidity mixing ratio</td>
<td width="16%" valign="TOP">kg/kg</td>
</tr>
<tr>
<td width="26%" valign="TOP">054</td>
<td width="58%" valign="TOP">Precipitable water </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">055</td>
<td width="58%" valign="TOP">Vapor pressure </td>
<td width="16%" valign="TOP">Pa</td>
</tr>
<tr>
<td width="26%" valign="TOP">056</td>
<td width="58%" valign="TOP">Saturation deficit </td>
<td width="16%" valign="TOP">Pa</td>
</tr>
<tr>
<td width="26%" valign="TOP">057</td>
<td width="58%" valign="TOP">Evaporation</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">058</td>
<td width="58%" valign="TOP">Cloud Ice</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">059</td>
<td width="58%" valign="TOP">Precipitation rate</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup>/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">060</td>
<td width="58%" valign="TOP">Thunderstorm probability</td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">061</td>
<td width="58%" valign="TOP">Total precipitation</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">062</td>
<td width="58%" valign="TOP">Large scale precipitation </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">063</td>
<td width="58%" valign="TOP">Convective precipitation </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">064</td>
<td width="58%" valign="TOP">Snowfall rate water equivalent</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup>s</td>
</tr>
<tr>
<td width="26%" valign="TOP">065</td>
<td width="58%" valign="TOP">Water equiv. of accum. snow depth </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">066</td>
<td width="58%" valign="TOP">Snow depth </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">067</td>
<td width="58%" valign="TOP">Mixed layer depth</td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">068</td>
<td width="58%" valign="TOP">Transient thermocline depth </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">069</td>
<td width="58%" valign="TOP">Main thermocline depth </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">070</td>
<td width="58%" valign="TOP">Main thermocline anomaly </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">071</td>
<td width="58%" valign="TOP">Total cloud cover</td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">072</td>
<td width="58%" valign="TOP">Convective cloud cover </td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">073</td>
<td width="58%" valign="TOP">Low cloud cover</td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">074</td>
<td width="58%" valign="TOP">Medium cloud cover </td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">075</td>
<td width="58%" valign="TOP">High cloud cover </td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">076</td>
<td width="58%" valign="TOP">Cloud water</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">077</td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">078</td>
<td width="58%" valign="TOP">Convective snow</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">079</td>
<td width="58%" valign="TOP">Large scale snow</td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">080</td>
<td width="58%" valign="TOP">Water Temperature </td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">081</td>
<td width="58%" valign="TOP">Land-sea mask</td>
<td width="16%" valign="TOP">Fraction</td>
</tr>
<tr>
<td width="26%" valign="TOP"></td>
<td width="58%" valign="TOP">(1=land;0=sea) (see note)</td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">082</td>
<td width="58%" valign="TOP">Deviation of sea level from mean </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">083</td>
<td width="58%" valign="TOP">Surface roughness </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">084</td>
<td width="58%" valign="TOP">Albedo</td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">085</td>
<td width="58%" valign="TOP">Soil temperature</td>
<td width="16%" valign="TOP">K</td>
</tr>
<tr>
<td width="26%" valign="TOP">086</td>
<td width="58%" valign="TOP">Soil moisture content </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup> </td>
</tr>
<tr>
<td width="26%" valign="TOP">087</td>
<td width="58%" valign="TOP">Vegetation </td>
<td width="16%" valign="TOP">%</td>
</tr>
<tr>
<td width="26%" valign="TOP">088</td>
<td width="58%" valign="TOP">Salinity</td>
<td width="16%" valign="TOP">kg/kg</td>
</tr>
<tr>
<td width="26%" valign="TOP">089</td>
<td width="58%" valign="TOP">Density</td>
<td width="16%" valign="TOP">kg/m<sup>3</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">090</td>
<td width="58%" valign="TOP">Water run off </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup> </td>
</tr>
<tr>
<td width="26%" valign="TOP">091</td>
<td width="58%" valign="TOP">Ice concentration </td>
<td width="16%" valign="TOP">Fraction</td>
</tr>
<tr>
<td width="26%" valign="TOP"></td>
<td width="58%" valign="TOP">(ice=1;no ice=0)(see note)</td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">092</td>
<td width="58%" valign="TOP">Ice thickness </td>
<td width="16%" valign="TOP">M</td>
</tr>
<tr>
<td width="26%" valign="TOP">093</td>
<td width="58%" valign="TOP">Direction of ice drift </td>
<td width="16%" valign="TOP">deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">094</td>
<td width="58%" valign="TOP">Speed of ice drift </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">095</td>
<td width="58%" valign="TOP">u-component of ice drift </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">096</td>
<td width="58%" valign="TOP">v-component of ice drift </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">097</td>
<td width="58%" valign="TOP">Ice growth rate </td>
<td width="16%" valign="TOP">m/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">098</td>
<td width="58%" valign="TOP">Ice divergence</td>
<td width="16%" valign="TOP">/s</td>
</tr>
<tr>
<td width="26%" valign="TOP">099</td>
<td width="58%" valign="TOP">Snow melt </td>
<td width="16%" valign="TOP">kg/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">100</td>
<td width="58%" valign="TOP">Significant height of combined wind </td>
<td width="16%" valign="TOP">m</td>
</tr>
<tr>
<td width="26%" valign="TOP"></td>
<td width="58%" valign="TOP">waves and swell</td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">101</td>
<td width="58%" valign="TOP">Direction of wind waves</td>
<td width="16%" valign="TOP">deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">102</td>
<td width="58%" valign="TOP">Significant height of wind waves </td>
<td width="16%" valign="TOP">m</td>
</tr>
<tr>
<td width="26%" valign="TOP">103</td>
<td width="58%" valign="TOP">Mean period of wind waves </td>
<td width="16%" valign="TOP">s </td>
</tr>
<tr>
<td width="26%" valign="TOP">104</td>
<td width="58%" valign="TOP">Direction of swell waves</td>
<td width="16%" valign="TOP">deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">105</td>
<td width="58%" valign="TOP">Significant height of swell waves </td>
<td width="16%" valign="TOP">m</td>
</tr>
<tr>
<td width="26%" valign="TOP">106</td>
<td width="58%" valign="TOP">Mean period of swell waves </td>
<td width="16%" valign="TOP">s </td>
</tr>
<tr>
<td width="26%" valign="TOP">107</td>
<td width="58%" valign="TOP">Primary wave direction </td>
<td width="16%" valign="TOP">deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">108</td>
<td width="58%" valign="TOP">Primary wave mean period </td>
<td width="16%" valign="TOP">s </td>
</tr>
<tr>
<td width="26%" valign="TOP">109</td>
<td width="58%" valign="TOP">Secondary wave direction </td>
<td width="16%" valign="TOP">deg. true</td>
</tr>
<tr>
<td width="26%" valign="TOP">110</td>
<td width="58%" valign="TOP">Secondary wave mean period</td>
<td width="16%" valign="TOP">s </td>
</tr>
<tr>
<td width="26%" valign="TOP">111</td>
<td width="58%" valign="TOP">Net short-wave radiation (surface) </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">112</td>
<td width="58%" valign="TOP">Net long wave radiation (surface) </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">113</td>
<td width="58%" valign="TOP">Net short-wave radiation (top of atmos.)</td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">114</td>
<td width="58%" valign="TOP">Net long wave radiation (top of atmos.) </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">115</td>
<td width="58%" valign="TOP">Long wave radiation </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">116</td>
<td width="58%" valign="TOP">Short wave radiation</td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">117</td>
<td width="58%" valign="TOP">Global radiation</td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">118 </td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">119 </td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">120 </td>
<td width="58%" valign="TOP"></td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">121</td>
<td width="58%" valign="TOP">Latent heat net flux </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">122</td>
<td width="58%" valign="TOP">Sensible heat net flux </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">123</td>
<td width="58%" valign="TOP">Boundary layer dissipation </td>
<td width="16%" valign="TOP">W/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">124</td>
<td width="58%" valign="TOP">Momentum flux, u component </td>
<td width="16%" valign="TOP">N/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">125</td>
<td width="58%" valign="TOP">Momentum flux, v component </td>
<td width="16%" valign="TOP">N/m<sup>2</sup></td>
</tr>
<tr>
<td width="26%" valign="TOP">126</td>
<td width="58%" valign="TOP">Wind mixing energy </td>
<td width="16%" valign="TOP">J</td>
</tr>
<tr>
<td width="26%" valign="TOP">127</td>
<td width="58%" valign="TOP">Image data</td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">128-254</td>
<td width="58%" valign="TOP">Reserved for use by originating center) </td>
<td width="16%" valign="TOP"></td>
</tr>
<tr>
<td width="26%" valign="TOP">255</td>
<td width="58%" valign="TOP">Missing</td>
<td width="16%" valign="TOP"></td>
</tr>
</tbody></table>
<p>&nbsp;Notes:
</p><ol>
<li>&nbsp;By convention, downward net fluxes of radiation or other quantities are assigned
negative values; upward net fluxes of radiation or other quantities are assigned positive
values.</li>
<li>Unidirectional flux values, where the direction of flow is indicated in the name of the
parameter shall all have positive values irrespective of the direction of flow. Net
(vertical) fluxes shall be calculated by subtracting the downward flux values from the
upward flux values</li>
<li>The u and v components of vector quantities are defined with reference to GDS Octet 17
and Table 7. However, if the GDS is <b>not</b> included in a message, then any wind
components are assumed to be resolved relative to the grid specified in the PDS with u and
v defined as positive in the direction of increasing x and y (or i and j) coordinates
respectively.</li>
<li>Provision is made for three types of spectra:</li>
<p>1) Direction and Frequency<br>
2) Direction and radial number<br>
3) Radial number and radial number</p>
<li>&nbsp;Parameters 81 and 91 show the units as "fraction", thus allowing for a
range of coverage. It is up to the user to employ the D (power of ten) scaling to assure
that the necessary precision is retained in the numeric values.</li>
</ol>
<p align="JUSTIFY"><font face="Univers">&nbsp;</font></p><font face="Univers">
<p align="CENTER">TABLE 3. TYPE AND VALUE OF LEVEL</p>
<p align="CENTER">(PDS Octets 10, 11, &amp; 12)</p>
</font>
<pre> Octet Number 10 Number 11 Number 12
VALUE MEANING CONTENTS
|---------------------------+-----------------+----------------|
| 0 - 99 special codes, | 0 | 0 |
| see Table 3a | | |
|---------------------------+-----------------+----------------|
| 100 isobaric level | pressure in hectoPascals (hPa) |
| | (2 octets) |
|---------------------------+-----------------+----------------|
| 101 layer between two | pressure of | pressure of |
| isobaric levels | top (kPa) | bottom (kPa) |
|---------------------------+-----------------+----------------|
| 102 mean sea level | 0 | 0 |
|---------------------------+-----------------+----------------|
| 103 fixed height level| height above mean sea level |
| | (MSL) in meters |
|---------------------------+-----------------+----------------|
| 104 layer between two | height of top | height of |
| height levels | (hm) | bottom (hm) |
| above msl | above mean sea level |
|---------------------------+-----------------+----------------|
| 105 fixed height above| height in meters |
| ground | (2 octets) |
|---------------------------+-----------------+----------------|
| 106 layer between two | height of top | height of |
| height levels | (hm) | bottom (hm) |
| above ground | above ground |
|---------------------------+-----------------+----------------|
| 107 sigma level | sigma value in 1/10000 |
| | (2 octets) |
|---------------------------+-----------------+----------------|
| 108 layer between two | sigma value at | sigma value |
| sigma levels | top | at bottom |
| | in 1/100 | in 1/100 |
|---------------------------+-----------------+----------------|
| 109 Hybrid level | level number |
| | (2 octets) |
|---------------------------+-----------------+----------------|
| 110 layer between two | level number of | level number |
| hybrid levels | top | of bottom |
|---------------------------+-----------------+----------------|
| 111 depth below | centimeters |
| land surface | (2 octets) |
|---------------------------+-----------------+----------------|
| 112 layer between two | depth of upper | depth of lower |
| depths below | surface (cm) | surface (cm) |
| land surface | | |
|---------------------------+-----------------+----------------|
| 113 isentropic | Potential Temperature K |
| (theta) level | (2 octets) |
|---------------------------+-----------------+----------------|
| 114 layer between two | 475K minus | 475K minus |
| isentropic | theta of top |theta of bottom |
| levels | in K | in K |
|---------------------------+-----------------+----------------|
| 121 layer between two | 1100 hPa minus | 1100 hPa minus |
| isobaric surfaces | pressure of | pressure of |
| (high precision) | top, in hPa | bottom, in hPa |
|---------------------------+-----------------+----------------|
| 125 Height level | centimeters |
| above ground | (2 octets) |
| (high precision) | |
|---------------------------+-----------------+----------------|
| 128 layer between two | 1.1 minus sigma| 1.1 minus sigma|
| sigma levels | of top, in | of bottom, in |
|---------------------------+-----------------+----------------|
| (high precision) | 1/1000 of sigma| 1/1000 of sigma|
|---------------------------+-----------------+----------------|
| 141 layer between two | pressure of | 1100hPa minus |
| isobaric surfaces | top, in kPa | pressure of |
| (mixed precision) | | bottom, in hPa |
|---------------------------+-----------------+----------------|
| 160 depth below | meters |
| sea level | (2 octets) |
|---------------------------+-----------------+----------------|
| 200 entire atmosphere | 0 |
| considered as a single layer (2 octets) |
|---------------------------+-----------------+----------------|
| 201 entire ocean | 0 |
| considered as a single layer (2 octets) |
|---------------------------+-----------------+----------------|</pre>
<dir>
<dir>
<font face="Univers"></font><p><font face="Univers"></font>Note: The numbering allows for additions within this
framework:</p><dir>
<dir>
<p>100-119 normal precision</p>
<p>120-139 high precision</p>
<p>140-159 mixed precision</p>
<p>&nbsp;</p>
</dir>
</dir>
</dir>
</dir>
<p align="CENTER">TABLE 3a. SPECIAL LEVELS</p>
<p align="CENTER">(PDS Octet 10)</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="475">
<tbody><tr>
<td width="17%" valign="TOP">VALUE</td>
<td width="83%" valign="TOP">LEVEL</td>
</tr>
<tr>
<td width="17%" valign="TOP"></td>
<td width="83%" valign="TOP"></td>
</tr>
<tr>
<td width="17%" valign="TOP">00</td>
<td width="83%" valign="TOP">reserved</td>
</tr>
<tr>
<td width="17%" valign="TOP">01</td>
<td width="83%" valign="TOP">surface (of the Earth, which includes sea surface)</td>
</tr>
<tr>
<td width="17%" valign="TOP">02</td>
<td width="83%" valign="TOP">cloud base level</td>
</tr>
<tr>
<td width="17%" valign="TOP">03</td>
<td width="83%" valign="TOP">cloud top level</td>
</tr>
<tr>
<td width="17%" valign="TOP">04</td>
<td width="83%" valign="TOP">0 deg (C) isotherm level</td>
</tr>
<tr>
<td width="17%" valign="TOP">05</td>
<td width="83%" valign="TOP">adiabatic condensation level(parcel lifted from surface)</td>
</tr>
<tr>
<td width="17%" valign="TOP">06</td>
<td width="83%" valign="TOP">maximum wind speed level</td>
</tr>
<tr>
<td width="17%" valign="TOP">07</td>
<td width="83%" valign="TOP">tropopause level</td>
</tr>
<tr>
<td width="17%" valign="TOP">08</td>
<td width="83%" valign="TOP">Nominal top of atmosphere</td>
</tr>
<tr>
<td width="17%" valign="TOP">09</td>
<td width="83%" valign="TOP">Sea bottom</td>
</tr>
<tr>
<td width="17%" valign="TOP">10-99</td>
<td width="83%" valign="TOP">Reserved</td>
</tr>
</tbody></table>
</center></div>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE 4. FORECAST TIME UNIT</p>
<p align="CENTER">(PDS Octet 18)</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="282">
<tbody><tr>
<td width="31%" valign="TOP">VALUE</td>
<td width="69%" valign="TOP">TIME UNIT</td>
</tr>
<tr>
<td width="31%" valign="TOP"></td>
<td width="69%" valign="TOP"></td>
</tr>
<tr>
<td width="31%" valign="TOP">0</td>
<td width="69%" valign="TOP">minute</td>
</tr>
<tr>
<td width="31%" valign="TOP">1</td>
<td width="69%" valign="TOP">hour</td>
</tr>
<tr>
<td width="31%" valign="TOP">2</td>
<td width="69%" valign="TOP">day</td>
</tr>
<tr>
<td width="31%" valign="TOP">3</td>
<td width="69%" valign="TOP">month</td>
</tr>
<tr>
<td width="31%" valign="TOP">4</td>
<td width="69%" valign="TOP">year</td>
</tr>
<tr>
<td width="31%" valign="TOP">5</td>
<td width="69%" valign="TOP">decade</td>
</tr>
<tr>
<td width="31%" valign="TOP">6</td>
<td width="69%" valign="TOP">normal (30 years)</td>
</tr>
<tr>
<td width="31%" valign="TOP">7</td>
<td width="69%" valign="TOP">century</td>
</tr>
<tr>
<td width="31%" valign="TOP">8-253</td>
<td width="69%" valign="TOP">reserved</td>
</tr>
<tr>
<td width="31%" valign="TOP">254</td>
<td width="69%" valign="TOP">second</td>
</tr>
</tbody></table>
</center></div>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE 5. </p>
<p align="CENTER">TIME RANGE INDICATOR</p>
<p align="CENTER">(PDS Octet 21)</p>
<table cellspacing="0" border="0" cellpadding="7" width="601">
<tbody><tr>
<td width="18%" valign="TOP">VALUE</td>
<td width="82%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">0</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Forecast product valid at reference time +
P1 P1&gt;0), or</p>
<p align="JUSTIFY">Uninitialized analysis product for reference time (P1=0). Or<br>
Image product for reference time (P1=0)</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">1</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Initialized analysis product for reference
time (P1=0).</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">2</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Product with a valid time ranging between
reference time + P1 and reference time + P2</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">3</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Average(reference time + P1 to reference
time + P2)</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">4</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Accumulation (reference time + P1 to
reference time + P2)</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">product considered valid at reference time
+ P2</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">5</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Difference(reference time + P2 minus
reference time + P1) product considered valid at reference time + P2</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">6-9</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">reserved</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">10</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">P1 occupies octets 19 and 20; </p></td>
</tr>
<tr>
<td width="18%" valign="TOP"></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">product valid at reference time + P1</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">11-50</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">reserved</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">51</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Climatological Mean Value: multiple year
averages of quantities which are themselves means over some period of time (P2) less than
a year. The reference time (R) indicates the date and time of the start of a period of
time, given by R to R + P2, over which a mean is formed; N indicates the number of such
period-means that are averaged together to form the climatological value, assuming that
the N period-mean fields are separated by one year. The reference time indicates the start
of the N-year climatology. N is given in octets 22-23 of the PDS.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">If P1 = 0 then the data averaged in the
basic interval P2 are assumed to be continuous, i.e., all available data are simply
averaged together.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">If P1 = 1 (the units of time - octet 18,
code table 4 - are not relevant here) then the data averaged together in the basic
interval P2 are valid only at the time (hour, minute) given in the reference time, for all
the days included in the P2 period. The units of P2 are given by the contents of octet 18
and Table 4.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">52-112</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">reserved</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">113</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Average of N forecasts (or initialized
analyses); each product has forecast period of P1 (P1=0 for initialized analyses);
products have reference times at intervals of P2, beginning at the given reference time.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">114</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Accumulation of N forecasts (or
initialized analyses); each product has forecast period of P1 (P1=0 for initialized
analyses); products have reference times at intervals of P2, beginning at the given
reference time.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">115</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Average of N forecasts, all with the same
reference time; the first has a forecast period of P1, the remaining forecasts follow at
intervals of P2.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">116</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Accumulation of N forecasts, all with the
same reference time; the first has a forecast period of P1, the remaining follow at
intervals of P2.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">117</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Average of N forecasts, the first has a
period of P1, the subsequent ones have forecast periods reduced from the previous one by
an interval of P2; the reference time for the first is given in octets 13-17, the
subsequent ones have reference times increased from the previous one by an interval of P2.
Thus all the forecasts have the same valid time, given by the initial reference time + P1.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">118</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Temporal variance, or covariance, of N
initialized analyses; each product has forecast period P1=0; products have reference times
at intervals of P2, beginning at the given reference time.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">119 -122</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">reserved</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">123</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Average of N uninitialized analyses,
starting at the reference time, at intervals of P2.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">124</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">Accumulation of N uninitialized analyses,
starting at the reference time, at intervals of P2.</p></td>
</tr>
<tr>
<td width="18%" valign="TOP"><p align="JUSTIFY">125-254</p></td>
<td width="82%" valign="TOP"><p align="JUSTIFY">reserved</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp;NOTES:</p>
<ol>
<li>For analysis products, or the first of a series of analysis products, the reference time
(octets 13 to 17) indicates the valid time.</li>
<li>For forecast products, or the first of a series of forecast products, the reference time
indicates the valid time of the analysis upon which the (first) forecast is based.</li>
<li>Initialized analysis products are allocated numbers distinct from those allocated to
uninitialized analysis products.</li>
<li>A value of 10 allows the period of a forecast to be extended over two octets; this
accommodates extended range forecasts.</li>
<li>Where products or a series of products are averaged or accumulated, the number involved
is to be represented in octets 22-23 of Section 1, while any number missing is to be
represented in octet 24.</li>
<li>Forecasts of the accumulation or difference of some quantity (e.g. quantitative
precipitation forecasts), indicated by values of 4 or 5 in octet 21, have a product valid
time given by the reference time + P2; the period of accumulation, or difference, can be
calculated as P2 - P1.</li>
</ol>
<dir>
<dir>
<p align="JUSTIFY">&nbsp;</p>
</dir>
</dir>
<p align="JUSTIFY">A few examples may help to clarify the use of Table 5: </p>
<p align="JUSTIFY">&nbsp; For analysis products P1 is zero and the time range indicator is
also zero; for initialized products (sometimes called "zero hour forecasts") P1
is zero, but octet 21 is set to 1. </p>
<p align="JUSTIFY">For forecasts, typically, P1 contains the number of hours of the
forecast (the unit indicator given in octet 18 would be 1) and octet 21 contains a zero. </p>
<p align="JUSTIFY">Value 51 allows for the identification of the most common
climatological entities. With P1=0, it could represent (or identify) the multiple year
climatology of anything from daily means (or less) to semi-annual means (or more, up to a
full year). The assumption is that all the available values within the basic period P2 are
averaged together. (An "annual mean climatology" would just be an average over
the total climatological period - Table 5, entry 3.) P1=1 allows for a diurnal
sub-stratification of the data within the P2 period, such as 30-year climatology of
February mean 00Z temperature starting at a date certain, or all the 12Z surface radiation
fluxes averaged for all the days in a season, or whatever. If other sub-stratifications
are appropriate they could be identified by different values of P1. No others have been
proposed at this time.</p>
<p align="JUSTIFY">Value 115 would be used, typically, for multiple day mean forecasts,
all derived from the same initial conditions.</p>
<p align="JUSTIFY">Value 117 would be used, typically, for Monte Carlo type calculations:
many forecasts valid at the same time from different initial (reference) times.</p>
<p align="JUSTIFY">Averages, accumulations, and differences get a somewhat specialized
treatment. If octet 21 (Table 5) has a value between 2 and 5 (inclusive) then the
reference time + P1 is the initial date/time and the reference time + P2 is the final
date/time of the period over which averaging or accumulation takes place. If, however,
octet 21 has a value of 113, 114, 115, 116, 117, 118, 123, or 124 then P2 specifies the
time interval between each of the fields (or the forecast initial times) that have been
averaged or accumulated. These latter values of octet 21 require the quantities averaged
to be equally separated in time; the former values, 3 and 4 in particular, allow for
irregular or unspecified intervals of time between the fields that are averaged or
accumulated.</p>
<b>
<p align="JUSTIFY"><a name="Section2">SECTION 2</a>: GRID DESCRIPTION SECTION (GDS)</p>
</b>
<p align="JUSTIFY">&nbsp; The purpose of the (optional) GDS is to provide a grid
description for grids not defined by number in Table 3.</p>
<table cellspacing="0" border="0" cellpadding="7" width="577">
<tbody><tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">&nbsp;Octet no.GDS Content</p></td>
<td width="73%"></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">1 - 3</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">Length in octets of the Grid Description
Section</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">4</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">NV, the number of vertical coordinate
parameters</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">5</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">PV, the location (octet number) of the
list of vertical coordinate parameters, if present or</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">PL, the location (octet number) of the
list of numbers of points in each row (when no vertical parameters are present), if
present</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">or</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">255</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">(all bits set to 1) if neither are present</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">6</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">Data representation type (See Table 6)</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">7 - 32</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">Grid description, according to data
representation type, except Lambert, Mercator or Space View.</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">or</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">7 - 42</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">Grid description for Lambert or Mercator
grid</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">or</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">7 - 44</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">Grid description for Space View
perspective grid</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">PV</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">List of vertical coordinate parameters</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">(length = NV x 4 octets);</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">if present, then PL = 4 x NV + PV</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"><p align="JUSTIFY">PL</p></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">List of numbers of points in each row,
used for quasi-regular grids</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"><p align="JUSTIFY">(length = NROWS x 2 octets, where NROWS is
the total number of rows defined within the grid description)</p></td>
</tr>
<tr>
<td width="27%" valign="TOP"></td>
<td width="73%" valign="TOP"></td>
</tr>
</tbody></table>
<dir>
<dir>
<p align="JUSTIFY">Note: NV and PV relate to features of GRIB not, at present, in use in
international exchange. See the WMO Manual on Codes for the descriptions of those
features.</p>
<dir>
<dir>
<dir>
<dir>
</dir>
</dir>
</dir>
</dir>
<p align="JUSTIFY">PL is used for "quasi-regular" or "thinned" grids;
e.g., a lat/lon grid where the number of points in each row is reduced as one moves
poleward from the equator. The reduction usually follows some mathematical formula
involving the cosine of the latitude, to generate an (approximately) equally spaced grid
array. The association of the numbers in octet PL (and following) with the particular row
follows the scanning mode specification in Table 8.</p>
<p align="JUSTIFY">&nbsp;</p>
</dir>
</dir>
<b>
<p align="CENTER">TABLES FOR THE GDS</p>
</b>
<p align="CENTER">TABLE 6. DATA REPRESENTATION TYPE</p>
<p align="CENTER">(GDS Octet 6)</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="487">
<tbody><tr>
<td width="16%" valign="TOP">VALUE</td>
<td width="84%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="16%" valign="TOP">0</td>
<td width="84%" valign="TOP">Latitude/Longitude Grid also called Equidistant Cylindrical
or Plate Carree projection grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">1</td>
<td width="84%" valign="TOP">Mercator Projection Grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">2</td>
<td width="84%" valign="TOP">Gnomonic Projection Grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">3</td>
<td width="84%" valign="TOP">Lambert Conformal, secant or tangent, conical or bipolar
(normal or oblique) Projection Grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">4</td>
<td width="84%" valign="TOP">Gaussian Latitude/Longitude Grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">5</td>
<td width="84%" valign="TOP">Polar Stereographic Projection Grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">6 - 12</td>
<td width="84%" valign="TOP">(reserved - see Manual on Codes)</td>
</tr>
<tr>
<td width="16%" valign="TOP">13</td>
<td width="84%" valign="TOP">Oblique Lambert conformal, secant or</td>
</tr>
<tr>
<td width="16%" valign="TOP"></td>
<td width="84%" valign="TOP">tangent, conical or bipolar,</td>
</tr>
<tr>
<td width="16%" valign="TOP"></td>
<td width="84%" valign="TOP">projection</td>
</tr>
<tr>
<td width="16%" valign="TOP">14 - 49</td>
<td width="84%" valign="TOP">(reserved - see Manual on Codes)</td>
</tr>
<tr>
<td width="16%" valign="TOP">50</td>
<td width="84%" valign="TOP">Spherical Harmonic Coefficients</td>
</tr>
<tr>
<td width="16%" valign="TOP">51 - 89</td>
<td width="84%" valign="TOP">(reserved - see Manual on Codes)</td>
</tr>
<tr>
<td width="16%" valign="TOP">90</td>
<td width="84%" valign="TOP">Space view perspective or orthographic grid</td>
</tr>
<tr>
<td width="16%" valign="TOP">91 - 254</td>
<td width="84%" valign="TOP">(reserved - see Manual on Codes)</td>
</tr>
</tbody></table>
</center></div>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE C. Sundry Grid Definitions</p>
<p align="CENTER">LATITUDE/LONGITUDE GRIDS</p>
<p align="CENTER">INCLUDING GAUSSIAN</p>
<p align="CENTER">(GDS Octets 7 - 32)</p>
<table cellspacing="0" border="0" cellpadding="7" width="601">
<tbody><tr>
<td width="21%" valign="TOP">OCTET NO.</td>
<td width="79%" valign="TOP">CONTENT &amp; MEANING</td>
</tr>
<tr>
<td width="21%" valign="TOP">7 - 8</td>
<td width="79%" valign="TOP">Ni - No. of points along a latitude circle</td>
</tr>
<tr>
<td width="21%" valign="TOP">9 - 10</td>
<td width="79%" valign="TOP">Nj - No. of points along a longitude meridian</td>
</tr>
<tr>
<td width="21%" valign="TOP">11 - 13</td>
<td width="79%" valign="TOP">La<sub>1</sub> - latitude of first grid point units:
millidegrees (degrees x 1000)<br>
values limited to range 0 - 90,000 bit 1 (leftmost) set to 1 for south latitude</td>
</tr>
<tr>
<td width="21%" valign="TOP">14 - 16</td>
<td width="79%" valign="TOP">Lo<sub>1</sub> - longitude of first grid point units:
millidegrees (degrees x 1000)<br>
values limited to range 0 - 360,000 bit 1 (leftmost) set to 1 for west longitude</td>
</tr>
<tr>
<td width="21%" valign="TOP">17</td>
<td width="79%" valign="TOP">Resolution and component flags (Table 7)</td>
</tr>
<tr>
<td width="21%" valign="TOP">18 - 20</td>
<td width="79%" valign="TOP">La<sub>2</sub> - Latitude of last grid point</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">(same units, value range, and bit 1 as La<sub>1</sub>)</td>
</tr>
<tr>
<td width="21%" valign="TOP">21 - 23</td>
<td width="79%" valign="TOP">Lo<sub>2</sub> - Longitude of last grid point</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">(same units, value range, and bit 1 as Lo<sub>1</sub>)</td>
</tr>
<tr>
<td width="21%" valign="TOP">24 - 25</td>
<td width="79%" valign="TOP">Di - Longitudinal Direction Increment </td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">(same units as Lo<sub>1</sub>) (if not given, all bits set =
1)</td>
</tr>
<tr>
<td width="21%" valign="TOP">26 - 27</td>
<td width="79%" valign="TOP"><u>Regular Lat/Lon Grid</u>:</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">Dj - Latitudinal Direction Increment (same units as La<sub>1</sub>)</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">(if not given, all bits set = 1) or</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP"><u>Gaussian Grid</u>:</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">N - number of latitude circles between a pole and the equator</td>
</tr>
<tr>
<td width="21%" valign="TOP"></td>
<td width="79%" valign="TOP">Mandatory if Gaussian Grid specified </td>
</tr>
<tr>
<td width="21%" valign="TOP">28</td>
<td width="79%" valign="TOP">Scanning mode flags (See Table 8)</td>
</tr>
<tr>
<td width="21%" valign="TOP">29 - 32</td>
<td width="79%" valign="TOP">Reserved (set to zero)</td>
</tr>
</tbody></table>
<p>&nbsp;Notes:
</p><ol>
<li>The latitude and longitude of the first and last grid points should always be given, for
regular grids.</li>
<li>If a quasi-regular grid is to be described, in which all the rows or columns do not
necessarily have the same number of grid points, either Ni (octets 7-8) or Nj (octets
9-10) and the corresponding Di (octets 24-25) or Dj (octets 26-27) shall be coded with all
bits set to 1 (missing).</li>
<li>A quasi-regular grid can be defined only for rows or columns, but not both
simultaneously. The first point in each row (column) shall be positioned at the meridian
(parallel) indicated in octets 11-16. The grid points shall be evenly spaced in latitude
(longitude).</li>
<li>For Gaussian grids only the rows can be rendered quasi-regular; the first point shall be
located at the meridian given in octets 14-16 and the last point at the meridian given in
octets 21-23.</li>
</ol>
<dir>
<dir>
<p align="JUSTIFY">&nbsp;</p>
</dir>
</dir>
<p align="CENTER">POLAR STEREOGRAPHIC GRIDS</p>
<p align="CENTER">(GDS Octets 7 - 32)</p>
<table cellspacing="0" border="0" cellpadding="7" width="581">
<tbody><tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">OCTET NO.</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">CONTENT &amp; MEANING</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">7 - 8</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Nx - Number of points along x-axis</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">9 - 10</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Ny - Number of points along y-axis</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">11 - 13</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">La1 - Latitude of first grid point</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">14 - 16</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Lo1 - Longitude of first grid point</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">17</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Resolution and component flags (see Table
7)</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">18 - 20</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Lov - The orientation of the grid; i.e.,
the east longitude value of the meridian which is parallel to the y-axis (or columns of
the grid) along which latitude increases as the y-coordinate increases. (Note: The
orientation longitude may, or may not, appear within a particular grid.)</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">21 - 23</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Dx - the X-direction grid length (see Note
2)</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">24 - 26</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Dy - the Y-direction grid length (see note
2)</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">27</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Projection center flag (see note 5)</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">28</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Scanning mode (see Table 8)</p></td>
</tr>
<tr>
<td width="21%" valign="TOP"><p align="JUSTIFY">29 - 32</p></td>
<td width="79%" valign="TOP"><p align="JUSTIFY">Set to 0 (reserved)</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">Notes:</p>
<ol>
<li>Latitude and longitude are in millidegrees (thousandths)</li>
<li>Grid lengths are in units of meters, at the 60 degree latitude circle nearest to the
pole in the projection plane.</li>
<li>Latitude values are limited to the range 0 - 90,000. Bit 1 is set to 1 to indicate south
latitude.</li>
<li>Longitude values are limited to the range 0 - 360,000. Bit one is set to 1 to indicate
west longitude.</li>
<li>Octet 27:</li>
<p align="JUSTIFY">Bit 1 set to 0 if the North pole is on the projection plane.</p>
<p align="JUSTIFY">Bit 1 set to 1 if the South pole is on the projection plane.</p>
<li>The first and last grid points may not necessarily be the same as the first and last
data points if the bit map section (BMS) is used.</li>
<li>The resolution flag (bit 1 of Table 7) is not applicable.</li>
</ol>
<dir>
<dir>
<p>&nbsp;</p>
<p align="CENTER">LAMBERT CONFORMAL SECANT OR TANGENT CONE GRIDS</p>
</dir>
</dir>
<p align="CENTER">(GDS Octets 7 - 42)</p>
<table cellspacing="0" border="0" cellpadding="7" width="590">
<tbody><tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">OCTET NO.</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">CONTENT &amp; MEANING</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">7 - 8</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Nx - Number of points along x-axis</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">9 - 10</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Ny - Number of points along y-axis</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">11 - 13</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">La1 - Latitude of first grid point</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">14 - 16</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Lo1 - Longitude of first grid point</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">17</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Resolution and component flags (see Table
7)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">18 - 20</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Lov - The orientation of the grid; i.e.,
the east longitude value of the meridian which is parallel to the y-axis (or columns of
the grid) along which latitude increases as the y-coordinate increases. (Note: The
orientation longitude may, or may not, appear within a particular grid.)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">21 - 23</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Dx - the X-direction grid length (see note
2)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">24 - 26</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Dy - the Y-direction grid length (see Note
2)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">27</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Projection center flag (see note 5)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">28</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Scanning mode (see Table 8)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">29 - 31</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Latin 1 - The first latitude from the pole
at which the secant cone cuts the spherical earth. (See Note 8)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">32 - 34</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Latin 2 - The second latitude from the
pole at which the secant cone cuts the spherical earth. (See Note 8)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">35 - 37</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Latitude of southern pole (millidegrees)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">38 - 40</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Longitude of southern pole (millidegrees)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">41 - 42</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Reserved (set to 0)</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">Notes:</p>
<ol>
<li>Latitude and longitude are in millidegrees (thousandths)</li>
<li>Grid lengths are in units of meters, at the intersection latitude circle nearest to the
pole in the projection plane.</li>
<li>Latitude values are limited to the range 0 - 90,000. Bit 1 is set to 1 to indicate south
latitude.</li>
<li>Longitude values are limited to the range 0 - 360,000. Bit one is set to 1 to indicate
west longitude.</li>
<li>Octet 27:<br>
Bit 1 set to 0 if the North pole is on the projection plane.<br>
Bit 1 set to 1 if the South pole is on the projection plane.<br>
Bit 2 set to 0 if only one projection center used<br>
Bit 2 set to 1 if projection is bipolar and symmetric</li>
<li>The first and last grid points may not necessarily be the same as the first and last
data points if the bit map section (BMS) is used.</li>
<li>The resolution flag (bit 1 of Table 7) is not applicable.</li>
<li>If Latin 1 = Latin 2 then the projection is on a tangent cone.</li>
</ol>
<p align="CENTER">MERCATOR GRIDS</p>
<p align="CENTER">(GDS Octets 7 - 42)</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="502">
<tbody><tr>
<td width="25%" valign="TOP">&nbsp;OCTET NO.</td>
<td width="75%" valign="TOP">CONTENT &amp; MEANING</td>
</tr>
<tr>
<td width="25%" valign="TOP">7 - 8</td>
<td width="75%" valign="TOP">Ni - Number of points along a latitude circle</td>
</tr>
<tr>
<td width="25%" valign="TOP">9 - 10</td>
<td width="75%" valign="TOP">Nj - Number of points along a longitude meridian</td>
</tr>
<tr>
<td width="25%" valign="TOP">11 - 13</td>
<td width="75%" valign="TOP">La1 - Latitude of first grid point</td>
</tr>
<tr>
<td width="25%" valign="TOP">14 - 16</td>
<td width="75%" valign="TOP">Lo1 - Longitude of first grid point</td>
</tr>
<tr>
<td width="25%" valign="TOP">17</td>
<td width="75%" valign="TOP">Resolution and component flags (see Table 7)</td>
</tr>
<tr>
<td width="25%" valign="TOP">18 - 20</td>
<td width="75%" valign="TOP">La2 - latitude of last grid point</td>
</tr>
<tr>
<td width="25%" valign="TOP">21 - 23</td>
<td width="75%" valign="TOP">Lo2 - longitude of last grid point</td>
</tr>
<tr>
<td width="25%" valign="TOP">24 - 26</td>
<td width="75%" valign="TOP">Latin - The latitude(s) at which the Mercator projection
cylinder intersects the earth. </td>
</tr>
<tr>
<td width="25%" valign="TOP">27</td>
<td width="75%" valign="TOP">Reserved (set to 0)</td>
</tr>
<tr>
<td width="25%" valign="TOP">28</td>
<td width="75%" valign="TOP">Scanning mode (see Table 8)</td>
</tr>
<tr>
<td width="25%" valign="TOP">29 - 31</td>
<td width="75%" valign="TOP">Di - the longitudinal direction increment (see Note 2)</td>
</tr>
<tr>
<td width="25%" valign="TOP">32 - 34</td>
<td width="75%" valign="TOP">Dj - the latitudinal direction increment (see note 2)</td>
</tr>
<tr>
<td width="25%" valign="TOP">35 - 42</td>
<td width="75%" valign="TOP">Reserved (set to 0)</td>
</tr>
</tbody></table>
</center></div>
<p align="JUSTIFY">Notes:</p>
<ol>
<li>Latitude and longitude are in millidegrees (thousandths)</li>
<li>Grid lengths are in units of meters, at the circle of latitude specified by Latin. </li>
<li>Latitude values are limited to the range 0 - 90,000. Bit 1 is set to 1 to indicate south
latitude.</li>
<li>Longitude values are limited to the range 0 - 360,000. Bit one is set to 1 to indicate
west longitude.</li>
<li>The latitude and longitude of the last grid point should always be given.</li>
<li>The first and last grid points may not necessarily be the same as the first and last
data points if the bit map section (BMS) is used.</li>
</ol>
<p align="CENTER">SPACE VIEW PERSPECTIVE OR ORTHOGRQAPHIC</p>
<p align="CENTER">(GDS Octets 7-44)</p>
<table cellspacing="0" border="0" cellpadding="7" width="571">
<tbody><tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">OCTET NUMBER</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">CONTENTS</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">7-8</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Nx - number of points along x axis
(columns)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">9-10</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Ny - number of points along y axis (rows
or lines)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">11-13</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Lap - latitude of sub-satellite point</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">14-16</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Lop - longitude of sub-satellite point</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">17</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Resolution and component flags (Table 7)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">18-20</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">dx - apparent diameter of earth in grid
lengths, in x direction</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">21-23</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">dy - apparent diameter of earth in grid
lengths, in y direction</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">24-25</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Xp - X-coordinate of sub satellite point</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">26-27</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Yp - Y-coordinate of sub-satellite point</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">28</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Scanning Mode (Table 8)</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">29-31</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">the orientation of the grid; i.e., the
angle in millidegrees between the increasing y axis and the meridian of the sub-satellite
point in the direction of increasing latitude (see Note 3).</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">32-34</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">Nr - the altitude of the camera from the
earth's center, measured in units of the earth's (equatorial) radius (See Note 4).</p></td>
</tr>
<tr>
<td width="17%" valign="TOP"><p align="JUSTIFY">35-44</p></td>
<td width="83%" valign="TOP"><p align="JUSTIFY">reserved</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp;Notes:</p>
<ol>
<li>It is assumed that the satellite is at its nominal position, i.e., it is looking
directly at its sub-satellite point.</li>
<li>Octet 32-34 shall be set to all ones (missing) to indicate the orthographic view (from
infinite distance).</li>
<li>It is the angle between increasing y axis and the meridian 180 degrees east if the
sub-satellite point is the North pole; or the meridian 0 degrees, if the sub-satellite
point is the south pole.</li>
<li>The apparent angular size of the earth will be given by 2 x asin(1/Nr).</li>
<li>The horizontal and vertical angular resolutions of he sensor (Rx and Ry), needed for
navigation equations, can be calculated from the following</li>
</ol>
<p align="CENTER">Rx = 2 x asin(1/Nr) / dx</p>
<p align="CENTER">Ry = 2 x asin(1/Nr) / dy</p>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">SPHERICAL HARMONIC COEFFICIENTS</p>
<p align="CENTER">(GDS Octets 7 - 32)</p>
<table cellspacing="0" border="0" cellpadding="7" width="520">
<tbody><tr>
<td width="28%" valign="TOP">OCTET NO.</td>
<td width="72%" valign="TOP">CONTENT &amp; MEANING</td>
</tr>
<tr>
<td width="28%" valign="TOP">7 - 8</td>
<td width="72%" valign="TOP">J - Pentagonal Resolution Parameter</td>
</tr>
<tr>
<td width="28%" valign="TOP">9 - 10</td>
<td width="72%" valign="TOP">K - Pentagonal Resolution Parameter</td>
</tr>
<tr>
<td width="28%" valign="TOP">11 - 12</td>
<td width="72%" valign="TOP">M - Pentagonal Resolution Parameter</td>
</tr>
<tr>
<td width="28%" valign="TOP">13</td>
<td width="72%" valign="TOP">Representation Type (See Table 9)</td>
</tr>
<tr>
<td width="28%" valign="TOP">14</td>
<td width="72%" valign="TOP">Coefficient Storage Mode (See Table 10)</td>
</tr>
<tr>
<td width="28%" valign="TOP">15 - 32</td>
<td width="72%" valign="TOP">Set to zero (reserved)</td>
</tr>
</tbody></table>
<p>&nbsp;&nbsp;</p>
<p align="CENTER">TABLE 7 - RESOLUTION AND COMPONENT FLAGS</p>
<p align="CENTER">(GDS Octet 17)</p>
<table cellspacing="0" border="0" cellpadding="7" width="576">
<tbody><tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">Bit</p></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">Value</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">Meaning</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">1</p></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">0</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">Direction increments not given</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">1</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">Direction increments given</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">2</p></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">0</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">Earth assumed spherical with radius =
6367.47 km</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">1</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">Earth assumed oblate spheroid with size</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="14%" valign="TOP"></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">as determined by IAU in 1965: </p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="14%" valign="TOP"></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">6378.160 km, 6356.775 km, f = 1/297.0</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">3-4</p></td>
<td width="14%" valign="TOP"></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">reserved (set to 0)</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">5</p></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">0</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">u- and v-components of vector quantities
resolved relative to easterly and northerly directions</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"></td>
<td width="14%" valign="TOP"><p align="JUSTIFY">1</p></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">u and v components of vector quantities
resolved relative to the defined grid in the direction of increasing x and y (or i and j)
coordinates respectively</p></td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">6-8</p></td>
<td width="14%" valign="TOP"></td>
<td width="74%" valign="TOP"><p align="JUSTIFY">reserved (set to 0)</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp;Note: If the GDS is <b>not</b> included in a message then any
wind components are assumed to be resolved relative to the grid specified in the PDS with
u and v defined as positive in the direction of increasing x and y (or i and j)
coordinates respectively.</p>
<p align="CENTER">&nbsp;</p>
<p align="CENTER">TABLE 8. SCANNING MODE FLAG</p>
<p align="CENTER">(GDS Octet 28)</p>
<div align="center"><center>
<table cellspacing="0" border="0" cellpadding="7" width="462">
<tbody><tr>
<td width="15%" valign="TOP">BIT</td>
<td width="17%" valign="TOP">VALUE</td>
<td width="69%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="15%" valign="TOP">1</td>
<td width="17%" valign="TOP">0</td>
<td width="69%" valign="TOP">Points scan in +i direction</td>
</tr>
<tr>
<td width="15%" valign="TOP"></td>
<td width="17%" valign="TOP">1</td>
<td width="69%" valign="TOP">Points scan in -i direction</td>
</tr>
<tr>
<td width="15%" valign="TOP">2</td>
<td width="17%" valign="TOP">0</td>
<td width="69%" valign="TOP">Points scan in -j direction</td>
</tr>
<tr>
<td width="15%" valign="TOP"></td>
<td width="17%" valign="TOP">1</td>
<td width="69%" valign="TOP">Points scan in +j direction</td>
</tr>
<tr>
<td width="15%" valign="TOP">3</td>
<td width="17%" valign="TOP">0</td>
<td width="69%" valign="TOP">Adjacent points in i direction are consecutive</td>
</tr>
<tr>
<td width="15%" valign="TOP"></td>
<td width="17%" valign="TOP"></td>
<td width="69%" valign="TOP">(FORTRAN: (I,J))</td>
</tr>
<tr>
<td width="15%" valign="TOP"></td>
<td width="17%" valign="TOP">1</td>
<td width="69%" valign="TOP">Adjacent points in j direction are consecutive</td>
</tr>
<tr>
<td width="15%" valign="TOP"></td>
<td width="17%" valign="TOP"></td>
<td width="69%" valign="TOP">(FORTRAN: (J,I))</td>
</tr>
</tbody></table>
</center></div>
<p align="JUSTIFY">&nbsp;Note: i direction is defined as west to east along a parallel of
latitude, or left to right along an x axis.</p>
<dir>
<dir>
<p align="JUSTIFY">j direction is defined as south to north along a meridian of longitude,
or bottom to top along a y axis.</p>
<p align="CENTER">TABLE 9. SPECTRAL REPRESENTATION TYPE</p>
</dir>
</dir>
<p align="CENTER">(GDS Octet 13)</p>
<table cellspacing="0" border="0" cellpadding="7" width="606">
<tbody><tr>
<td width="12%" valign="TOP">VALUE</td>
<td width="88%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="12%" valign="TOP"><p align="JUSTIFY">1</p></td>
<td width="88%" valign="TOP"><p align="JUSTIFY">Associated Legendre Polynomials of the
First Kind with normalization such that the integral equals 1</p></td>
</tr>
</tbody></table>
<p align="CENTER">TABLE 10. COEFFICIENT STORAGE MODE</p>
<p align="CENTER">(GDS Octet 14)</p>
<table cellspacing="0" border="0" cellpadding="7" width="616">
<tbody><tr>
<td width="13%" valign="TOP">VALUE</td>
<td width="87%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="13%" valign="TOP"><p align="JUSTIFY">1 </p></td>
<td width="87%" valign="TOP"><p align="JUSTIFY">The complex coefficients X<sub>nm</sub>
are stored for m <u>&gt;</u> 0 as pairs of real numbers Re(Xnm), Im(Xnm) ordered with n
increasing from m to N(m), first for m = 0 and then for m = 1, 2, 3,...M. The real part of
the (0,0) coefficient is stored in octets 12-15 of the BDS, as a floating point number in
the same manner as the packing reference value, with units as in Table 2. The remaining
coefficients, starting with the imaginary part of the (0,0) coefficient, are packed
according to the GRIB packing algorithm, with units as given in Table 5, in octets 16 and
onward in the BDS.</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp;&nbsp;NOTES ON SPECTRAL TRUNCATION:</p>
<dir>
<dir>
<p align="CENTER"><img src="./GuidetoGRIB_files/Guide-GRIB-2.gif" width="217" height="48"></p>
</dir>
</dir>
<p align="JUSTIFY">Using the associated Legendre Polynomials of the First Kind, Pnm, as
typical expansion functions, any variable x(l ,F), which is a function of longitude, l ,
and sin(latitude), F , can be represented by</p>
<p align="CENTER">&nbsp;</p>
<p align="JUSTIFY"><img src="./GuidetoGRIB_files/Guide-GRIB-3.gif" alt="Guide-GRIB-3.gif (2477 bytes)" align="left" hspace="4" width="287" height="248">In the summations, M is the
maximum zonal wave number that is to be included, and K &amp; J together define the
maximum meridional total wave number N(m), which, it should be noted, is a function of m.
A sketch shows the relationships:</p>
<p align="JUSTIFY">&nbsp;&nbsp;</p>
<p align="JUSTIFY">In this figure, the ordinate is n the zonal wave number, the abscissa,
m, is the total meridional wave number, the vertical line at m = M is the zonal
truncation, and the diagonal passing through (0,0) is the line n = m. The Legendre
Polynomials are defined only on or above this line, that is for n <u>&gt;</u> m. On the
n-axis, the horizontal line at n = K indicates the upper limit to n values, and the
diagonal that intersects the n-axis at n = J indicates the upper limit of the area in
which the Polynomials are defined. The shaded irregular pentagon defined by the n-axis,
the diagonal from n = J, the horizontal n = K, the vertical m = M, and the other diagonal
n = m surrounds the region of the (n x m) plane containing the Legendre Polynomials used
in the expansion.</p>
<p align="JUSTIFY">This general pentagonal truncation reduces to some familiar common
truncations as special cases:</p>
<p align="CENTER">Triangular: K = J = M and N(m) = J</p>
<p align="CENTER">Rhomboidal: K = J + M and N(m) = J + m</p>
<p align="CENTER">Trapezoidal: K = J, K &gt; M and N(m) = J</p>
<dir>
<dir>
<dir>
<dir>
<p align="JUSTIFY">&nbsp;</p>
</dir>
</dir>
</dir>
</dir>
<p align="JUSTIFY">In all of the above m can take on negative values to represent the
imaginary part of the spectral coefficients.</p>
<b>
<p align="JUSTIFY"><a name="Section3">SECTION 3</a>: BIT MAP SECTION (BMS)</p>
</b>
<p align="JUSTIFY">&nbsp; The purpose of the (optional) BMS is to provide either a bit map
or a reference to a bit map pre-defined by the center. The bit map consists of contiguous
bits with a bit-to-data-point correspondence as defined in the grid description. A bit set
equal to 1 implies the presence of a datum for that grid point in the BDS; a value of zero
implies the absence of such. This is useful in shrinking fields where fair portions of the
field are not defined. An example would be global grids of sea surface temperature; the
bit map would be used to suppress the "data" at grid points over land. One would
not want to use the BMS if the data were un-defined at only a small number of grid points
as the overhead of adding the bit map array (one bit for each grid point) might add more
bits to the overall message that were subtracted by the removal of a few data values.</p>
<table cellspacing="0" border="0" cellpadding="7" width="499">
<tbody><tr>
<td width="16%" valign="TOP"><p align="JUSTIFY">&nbsp;Octet no.</p></td>
<td width="84%" valign="TOP"></td>
</tr>
<tr>
<td width="16%" valign="TOP"><p align="JUSTIFY">1 - 3</p></td>
<td width="84%" valign="TOP"><p align="JUSTIFY">Length in octets of Bit Map Section</p></td>
</tr>
<tr>
<td width="16%" valign="TOP"><p align="JUSTIFY">4</p></td>
<td width="84%" valign="TOP"><p align="JUSTIFY">Number of unused bits at end of Section 3.</p></td>
</tr>
<tr>
<td width="16%" valign="TOP"><p align="JUSTIFY">5 - 6</p></td>
<td width="84%" valign="TOP"><p align="JUSTIFY">Numeric:</p></td>
</tr>
<tr>
<td width="16%" valign="TOP" height="51"></td>
<td width="84%" valign="TOP" height="51">= 0: a bit map follows;<br>
otherwise: the numeric refers to a predefined bit map provided by the center</td>
</tr>
<tr>
<td width="16%" valign="TOP"><p align="JUSTIFY">7 - nnn</p></td>
<td width="84%" valign="TOP"><p align="JUSTIFY">Bit map, zero filled to an even number of
octets</p></td>
</tr>
</tbody></table>
<b>
<p align="JUSTIFY"><a name="Section4">SECTION 4</a>: BINARY DATA SECTION (BDS).</p>
</b>
<p align="JUSTIFY">&nbsp; The BDS contains the packed data and the binary scaling
information needed to reconstruct the original data from the packed data. The required
decimal scale factor is found in the PDS, above. The data stream is zero filled to an even
number of octets. </p>
<table cellspacing="0" border="0" cellpadding="7" width="601">
<tbody><tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">&nbsp;Octet no.</p></td>
<td width="85%" valign="TOP"></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">1 - 3</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Length in octets of binary data section</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">4</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Bits 1 through 4: Flag - See Table 11</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Bits 5 through 8: Number of unused bits at
end of Section 4.</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">5 - 6</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">The binary scale factor (E). A negative
value is indicated by setting the high order bit (bit No. 1) in octet 5 to 1 (on).</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">7 - 10</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Reference value (minimum value); floating
point representation of the number.</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">11</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Number of bits into which a datum point is
packed</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">12 -nnn</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Variable, depending on octet 4; zero
filled to an even number of octets.</p></td>
</tr>
<tr>
<td width="15%" valign="TOP"><p align="JUSTIFY">14</p></td>
<td width="85%" valign="TOP"><p align="JUSTIFY">Optionally, may contain an extension of
the flags in octet 4. See Table 11.</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">&nbsp; Here are some of the various forms the binary data can take; the
flag table in BDS octet 4, possibly extended into octet 14, identifies which variant is in
use. </p>
<p align="JUSTIFY">&nbsp;<u>Grid-point data - Simple packing</u></p><u>
</u>
<p align="JUSTIFY">&nbsp; Here the data simply begin in octet 12 and continue, packed
according to the simple packing algorithm described above, without any particular regard
for computer "word" boundaries, until there is no more data. There may be some
"zero-fill" bits at the end.</p>
<p align="JUSTIFY">If all the data in a grid point field happen to have the same value,
then all of the deviations from the reference value are set to zero. Since a zero value
requires no bits for packing, octet 11 is set to zero, thus indicating a field of constant
data, the value of which is given by the reference value. Under these circumstances, octet
12 is set to zero (the required "zero fill to an even number of octets") and
bits 5-8 of octet 4 contain an 8. The number of data points in the field is implied by the
grid identification given in the PDS and/or the GDS and BMS.</p>
<p align="JUSTIFY">&nbsp;<u>Spherical Harmonic Coefficients - Simple packing</u></p><u>
</u>
<p align="JUSTIFY">&nbsp; Octets 12-15 contain the real part of the (0.0) coefficient in
the same floating point format as the reference value in octets 7-10. The imaginary part
of the (0.0) coefficient, mathematically, is always equal zero. Octets 16 to the end
contain the remaining coefficients packed up as binary data with the same sort of scaling,
reference value, and the like, as with grid-point numbers. Excluding the (0,0)
coefficient, which is usually much larger than the others, from the packing operation
means that the remaining coefficients can be packed to a given precision more efficiently
(fewer bits per word) than would be the case otherwise.</p>
<p align="JUSTIFY">&nbsp;<u>Grid-Point Data - Second Order or Complex Packing</u></p><u>
</u>
<p align="JUSTIFY">&nbsp; Before laying out where the various second order values,
sub-parameters, counters, and what have you, go, it is appropriate to describe the second
order packing method in an algorithmic manner.</p>
<p align="JUSTIFY">Referring back to the description of simple packing, the encoding
method is the same up to part way through the fourth step, stopping just short of the
actual packing of the scaled integers into the "words" of either a pre-specified
or calculated bit length.</p>
<p align="JUSTIFY">The basic outline of second order packing is to scan through the array
of integers (one per grid point, or possibly less than that if the Bit Map Section has
been employed to discard some of the null value points) and seek out sub-sections
exhibiting relatively low variability within the sub-section. One then finds the (local)
minimum value in that sub-section and subtracts it from the ("first order")
integers in that sub-section, which leave a set of "second order" integers.
These numbers are then scanned to find the maximum value, which in turn is used to specify
the minimum bit width for a "word" necessary to contain the sub-section set of
second order numbers. </p>
<p align="JUSTIFY">The term "first order" in this context refers to the integer
variables that result from subtracting the overall (global) minimum from the original
variables and then doing all scaling and rounding; "second order" refers to the
variables that result from subtracting the local minimum from the sub-set of first order
variables. No further scaling is necessary or appropriate.</p>
<p align="JUSTIFY">The sub-section set of numbers are then packed into "words"
of the just determined bit length. The overall savings in space comes about because the
second order values are, usually, smaller than their first order counterparts. They have,
after all, had two minima subtracted from the original values, the overall minimum and the
local minimum, where the first order values have had only the overall minimum subtracted
out. There is no guarantee, however, that the second order packing will compress a given
field to a greater degree than the first order packing. If the first order field of
integers is highly variable, or generally close to zero, then there will be no gain in
compression. But if the field shows long runs of small variation, particularly if some of
the runs are constant (zero variability), then the second order packing will contribute to
the compression. </p>
<p align="JUSTIFY">The process then repeats and a whole collection of sub-sections are
found, their local minima are subtracted, etc. One of the tricky parts of this process is
defining just what is meant by a "sub-section of low variability". The WMO
Manual is silent on this as it only describes how the sub-sections and their ancillary
data are to be packed in the message. The U.S. National Weather Service, the U.K.
Meteorological Office, the European Centre for Medium-Range Weather Forecasts, and
probably other groups have, independently, designed selection criteria and built them into
GRIB encoders. It is beyond the scope of this document to attempt to describe them in any
detail. These groups have all expressed their willingness to share their GRIB encoders
with any who ask for them.</p>
<p align="JUSTIFY">Before laying out where the second order values, etc., are placed in a
message, we had best review just what information has to be saved. We need to include the
following information:</p>
<dir>
<dir>
<p align="JUSTIFY">1) How many sub-sections there are;</p>
<p align="JUSTIFY">2) Where does each sub-section begin;</p>
<p align="JUSTIFY">3) Where does each sub-section end; or, how many data points are in
each sub-section;</p>
<p align="JUSTIFY">4) That is the local minimum value (a first order value) that was found
for each sub-section;</p>
<p align="JUSTIFY">5) What is the bit width of the collection of first order values (the
local minima) found for each sub-section;</p>
<p align="JUSTIFY">6) What are the second order values for each sub-section;</p>
<p align="JUSTIFY">7) What are the bit widths of the second order values appropriate for
all the sub-section; and, finally,</p>
<p align="JUSTIFY">8) Sufficient information to specify where the above information is
located.</p>
</dir>
</dir>
<p align="JUSTIFY">A moments consideration (a long moment, perhaps) will satisfy the
reader that the information given will be sufficient to reconstruct the original data
field.</p>
<p align="JUSTIFY">The information needed for points 2) and 3), the beginning and end of
the sub-sections, is presented in the form of a bit map, called a "secondary bit
map" to distinguish it from the bit map (optionally) contained in the BMS. There is
one bit for each grid point containing data, ordered in the same way as the grid is laid
out. The "primary" bit map, the BMS bit map, may have been used to eliminate
data at points where the data are meaningless - only the remaining "real" data
points are matched by the bits in the secondary bit map. This possibility is understood to
exist throughout the following discussion. The start of each sub-section is indicated by
the corresponding bit set to "on" or to a value of 1. Clearly, the first bit in
the secondary bit map will always be set on, since the first data point must be the start
of the first sub-section. (If it is not, then something is wrong somewhere. Unfortunately
it is not always easy to tell just where the error occurred.) The secondary bit map is
then no more than a collection of 1s and 0s, indicating the start and the extent of each
sub-section. It would be possible to scan through the secondary bit map and determine how
many sub-sections there are; however, this number is explicitly included in the GRIB
message to save one the trouble, and to serve as an internal self-checking mechanism.</p>
<p align="JUSTIFY">At long last, then, here is the layout of the information, with further
explanatory notes, when second order packing has been employed:</p>
<table cellspacing="0" border="0" cellpadding="7" width="607">
<tbody><tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">&nbsp;Octet no.Content</p></td>
<td width="86%" valign="MIDDLE"></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">1-3</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Length in octets of binary data section</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">4</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Bits 1 through 4: Flag - See Table 11</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Bits 5 through 8: Number of unused bits at
end of Section 4.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">5-6</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">The binary scale factor (E). A negative
value is indicated by setting the high order bit(bit No. 1) in octet 5 to 1 (on).</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">7-10</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Reference value (minimum value); floating
point representation of the number. This is the overall or "global" minimum that
has been subtracted from all the values.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">11</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Number of bits into which a datum point is
packed. This width now refers to the collection of first order packed values that serve as
the local minimum values, one for each sub-section. It is determined in the same manner as
for the simple (first order) packing.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">12-13</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">N1 - Octet number, relative to the start
of the BDS, at which the collection of first order packed numbers begins, i.e. the
collection of local minimum values.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">14</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">The flags that are an extension of octet
4. See Code Table 11.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">15-16</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">N2 - Octet number, relative to the start
of the BDS, at which the collection of second order packed numbers begins.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">17-18</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">P1 - The number of first order packed
values, the local minima. This number is the same as the number of sub-sections.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">19-20</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">P2 - The number of second order packed
values. This is the number of actual data points (as possibly modified by the bit map in
the BMS, if any).</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">21</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Reserved</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">22-(xx-1)</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Width(s), in bits, of the second order
packed values; each width value is contained in 1 octet. There are as many width values,
or octets, as there are sub-sections, P1 of them. However, there may be but one such value
under special circumstances; see below. Also, the width value for a particular sub-section
may perfectly well be zero.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">xx-(N1-1)</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">Secondary bit map, one bit for each data
point. It will be P2 bits long, then padded to an even number of octets with binary 0.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">N1-(N2-1)</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">P1 first order packed values, the local
minima, each held in a "word" of bit-length found in octet 11, then padded to a
whole number of octets with 0s.</p></td>
</tr>
<tr>
<td width="14%" valign="TOP"><p align="JUSTIFY">N2-...</p></td>
<td width="86%" valign="TOP"><p align="JUSTIFY">P2 second order packed values. There is no
"marking" of the sub-sections here; all the sub-section second order values are
placed in a continuous string of bits. The bit-length of the "words" holding the
values will change from place to place but again this has to be determined by reference to
the other information.</p></td>
</tr>
</tbody></table>
<p align="JUSTIFY">As usual, there may be padding by binary 0 bits sufficient to bring the
entire section to an even number of octets.</p>
<p align="JUSTIFY">&nbsp; There are a small number of special cases and variations on the
above layout:</p>
<p align="JUSTIFY">If the bit-width for a sub-section is zero, then no second order values
for that sub-section are included in the part of the message starting at octet N2. This
will happen is all the first order values in the sub-section are identical. This is a form
of "run-length encoding" and contributes greatly to packing efficiency if the
original data contains strings of constant value (including zero).</p>
<p align="JUSTIFY">Under some circumstances, it may turn out that there is no need to use
different bit-widths for each of the sub-sections. In that case, a flag is set in bit 8 of
the extended flags found in octet 14 (see table 11) indicating that all the sub-sections
are packed with the same bit-width, and that the single value will be found in octet 22.</p>
<p align="JUSTIFY">Row by row packing is defined as selecting entire rows (or columns) to
serve as sub-sections, without regard to "variability" determinations. It can
have some compression value. If row by row packing is employed, this is indicated by
setting a flag in bit 7 of the extended flags found in octet 14 (see table 11) and NOT
including the secondary bit map in the message. It is unnecessary since the length of the
rows (columns) is known from the grid specifications given elsewhere in the message.</p>
<b>
<p align="CENTER">TABLES FOR THE BDS</p>
</b>
<p align="CENTER">TABLE 11. FLAG</p>
<p align="CENTER">(BDS Octet 4 and, optionally, 14)</p>
<table cellspacing="0" border="0" cellpadding="7" width="541">
<tbody><tr>
<td width="10%" valign="TOP">BIT</td>
<td width="16%" valign="TOP">VALUE</td>
<td width="74%" valign="TOP">MEANING</td>
</tr>
<tr>
<td width="10%" valign="TOP">1</td>
<td width="16%" valign="TOP">0</td>
<td width="74%" valign="TOP">Grid point data</td>
</tr>
<tr>
<td width="10%" valign="TOP"></td>
<td width="16%" valign="TOP">1</td>
<td width="74%" valign="TOP">Spherical Harmonic Coefficients</td>
</tr>
<tr>
<td width="10%" valign="TOP">2</td>
<td width="16%" valign="TOP">0</td>
<td width="74%" valign="TOP">Simple packing </td>
</tr>
<tr>
<td width="10%" valign="TOP"></td>
<td width="16%" valign="TOP">1</td>
<td width="74%" valign="TOP">Second order ("Complex") Packing </td>
</tr>
<tr>
<td width="10%" valign="TOP">3</td>
<td width="16%" valign="TOP">0</td>
<td width="74%" valign="TOP">Original data were floating point values</td>
</tr>
<tr>
<td width="10%" valign="TOP"></td>
<td width="16%" valign="TOP">1</td>
<td width="74%" valign="TOP">Original data were integer values</td>
</tr>
<tr>
<td width="10%" valign="TOP">4</td>
<td width="16%" valign="TOP">0</td>
<td width="74%" valign="TOP">No additional flags at octet 14</td>
</tr>
<tr>
<td width="10%" valign="TOP"></td>
<td width="16%" valign="TOP">1</td>
<td width="74%" valign="TOP">Octet 14 contains flag bits 5 - 12</td>
</tr>
</tbody></table>
<p>&nbsp;The following gives the meaning of the bits in octet 14 ONLY if bit 4 is set to
1. Otherwise octet 14 contains regular binary data. </p>
<table cellspacing="0" border="0" cellpadding="7" width="571">
<tbody><tr>
<td width="11%" valign="TOP">&nbsp;5</td>
<td width="14%" valign="TOP"></td>
<td width="76%" valign="TOP">Reserved (set to 0)</td>
</tr>
<tr>
<td width="11%" valign="TOP">6</td>
<td width="14%" valign="TOP">0</td>
<td width="76%" valign="TOP">Single datum at each grid point</td>
</tr>
<tr>
<td width="11%" valign="TOP"></td>
<td width="14%" valign="TOP">1</td>
<td width="76%" valign="TOP">Matrix of values at each grid point</td>
</tr>
<tr>
<td width="11%" valign="TOP">7</td>
<td width="14%" valign="TOP">0</td>
<td width="76%" valign="TOP">No secondary bit maps</td>
</tr>
<tr>
<td width="11%" valign="TOP"></td>
<td width="14%" valign="TOP">1</td>
<td width="76%" valign="TOP">Secondary bit maps present</td>
</tr>
<tr>
<td width="11%" valign="TOP">8</td>
<td width="14%" valign="TOP">0</td>
<td width="76%" valign="TOP">Second order values have constant width</td>
</tr>
<tr>
<td width="11%" valign="TOP"></td>
<td width="14%" valign="TOP">1</td>
<td width="76%" valign="TOP">Second order values have different widths</td>
</tr>
<tr>
<td width="11%" valign="TOP">9-12</td>
<td width="14%" valign="TOP"></td>
<td width="76%" valign="TOP">Reserved (set to 0)</td>
</tr>
</tbody></table>
<p>&nbsp;Notes:
</p><ol>
<li>Bit 3 is set to 1 to indicate that the original data were integers; when this is the
case any non-zero reference values should be rounded to an integer value prior to placing
in the GRIB BDS.</li>
<li>Bit 4 is set to 1 to indicate that bits 5 to 12 are contained in octet 14 of the data
section.</li>
<li>Although GRIB is not capable of representing a matrix of data values at each grid point,
the meaning of bit 6 is retained in anticipation of a future capability.</li>
<li>When secondary bit maps are present in the data (used in association with second order
packing) this is indicated by setting bit 7 to 1.</li>
<li>When octet 14 contains the extended flag information, octets 12 and 13 will also contain
"special" information; the actual data will begin in a subsequent octet. See
above.</li>
</ol>
<b>
<p align="JUSTIFY"><a name="Section5">SECTION 5</a>: END SECTION.</p>
</b>
<p align="JUSTIFY">&nbsp; The end section serves a human readable indication of the end of
a GRIB record. It can also be used for computer verification that a complete GRIB record
is available for data extraction. It should not be used as a search target since a '7777'
bit combination could exist anywhere in the binary data stream.</p>
<p align="JUSTIFY">&nbsp;Octet no.</p>
<p align="JUSTIFY">1-4 '7777' (Coded CCITT-ITA No. 5)</p>
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