USERS GRIB1 GUIDE (FORTRAN 90) Contents: Introduction GRIB1 Encoding Routines GRIB1 Decoding Routine Extracting GRIB1 Fields from a GRIB1 file GRIB1 Tables GRIB1 Routine Docblocks
=============================================================================== Introduction This document briefly describes the routines available for encoding/decoding GRIB Edition 1 messages. A basic familiarity with GRIB is assumed. A GRIB message is a machine independent format for storing one or more gridded data fields. Each GRIB message consists of the following sections: SECTION SECTION SECTION SECTION SECTION SECTION 0 1 2 3 4 5 Indicator Section Product Definition Section (PDS) Grid Definition Section (GDS) Bit-map Section (Optional) Binary Data Section End Section
=============================================================================== GRIB1 Encoding Routines There are several routines that one can use to encode a GRIB1 message. Subroutine W3FI72 can be used to encode a GRIB1 message which is passed back to the calling program in a character array. It is the users responsibility to ensure that the character array that will hold the packed GRIB1 message has been allocated large enough prior to calling W3FI72. Another option is subroutine PUTGB. PUTGB encodes a GRIB1 message and writes it to a file. The message is not returned to the calling routine. The output GRIB1 data file must be opened with a call to subroutine BAOPEN (or BAOPENW) prior to the call to PUTGB. A call to BACLOSE is recommended at the end of the program to close the output file properly. Example usage: integer,dimension(200) :: KPDS,KGDS logical*1,allocatable :: LB(:) ! bitmap real,allocatable :: F(:) ! grid point data values lugb=50 ! Open GRIB1 file call baopenw(LUGB,"filename",iret) ! Set up bitmap and data field numpts=??????
allocate(LB(numpts)) allocate(F(numpts)) ! Set GRIB1 field identification values to encode KPDS(?)= KGDS(?)= ! pack and write field to file CALL PUTGB(LUGB,numpts,KPDS,KGDS,LB,F,iret) ! Close file ... call baclose(LUGB,iret) stop end There are other similar routines in the PUTGB family that can be used to encode GRIB1 messages and write them out to a file: PUTGBEX - Used to encode GRIB1 messages with NCEP PDS extensions to specify ensemble information. PUTGN - Allows users to specify a binary scale factor or limit amount of space each data point should occupy. Please see the "GRIB1 Routine Docblocks" section below for subroutine argument usage for the routines mentioned above. =============================================================================== GRIB1 Decoding Routine Subroutine W3FI63 can be used to decode a given GRIB1 message that resides in a character array in memory. This routine will return the unpacked values in the PDS and GDS, a bitmap array, and the unpacked grid point data values. It is the users responsibility to ensure that the returned arrays have been allocated large enough prior to calling W3FI63. Please see the "GRIB1 Routine Docblocks" section below for subroutine argument usage for the routine mentioned above. =============================================================================== Extracting GRIB1 Fields from a GRIB1 file Subroutine GETGB can be used to extract a specified field from a file containing many GRIB1 messages. GETGB searches an index to find the location of the user specified field. The index can be supplied from a separate GRIB1 index file, or it can be generated internally. The GRIB1 data file ( and the index file, if supplied ) must be opened with a call to subroutine BAOPEN prior to the call to GETGB. Users can request a particular field by specifying the PDS and GDS values that they wish to match. GETGB will return the PDS, GDS, bitmap, and grid point data values.
Example usage: integer,parameter :: MAXPTS=?????? integer,dimension(200) :: JPDS,JGDS,KPDS,KGDS logical*1,dimension(MAXPTS) :: LB ! bitmap real,dimension(MAXPTS) :: F ! grid point data values lugb=10 lugi=0 ! Open GRIB1 file call baopenr(LUGB,"filename",iret) ! Set GRIB1 field identification values to search for j=0 ! search from beginning jpds(?)= jgds(?)= ! Get field from file CALL GETGB(LUGB,LUGI,MAXPTS,J,JPDS,JGDS, & KF,K,KPDS,KGDS,LB,F,IRET) ! Process field ... firstval=F(1) lastval=F(KF) fldmax=maxval(F) fldmin=minval(F) stop end There are other similar routines in the GETGB family that can be used to extract data from a GRIB1 file: GETGBEX - Used to search for and decode GRIB1 messages using NCEP PDS ext...
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