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.
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. The changes were of such structural magnitude as to require a new Edition of GRIB, Edition 1, which this document describes. 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. 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 plan is to design a Unit 9 lo1 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. 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 Unit 9 lo1 editions of the code, and older versions of the parameter tables. 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 layerencoded as a continuous bit stream.
A GRIB record consists of six sections, two of which are optional: That way there will be no question about just what is the "correct" geographical grid for a particular field. Most centers require bulletin headers to enable them to receive, identify, and switch messages.
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.
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.
Some additional tables not found in the WMO Manual are indicated by letters.
These, generally, contain information unique to a particular processing center. 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.
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.
The representation of a single value is such that: The reference value R is the minimum value of the possibly decimally scaled data that is being encoded. R is placed in the Binary Data Section in four octets as a single precision floating-point number: The appropriate formula to recover the value of R is: 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.
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. 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.
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 "D" scaling and the power-of-2 "E" scaling.
Just how this comes about will be described momentarily.
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