GEMPAK map files are stored as direct access, packed binary files in a GEMPAK Standard Format (GSF). This form was chosen for compactness and relatively rapid access.
In addition, a Sequential Standard Format (SSF) is defined for storing maps as ASCII sequential files. SSF files are used to create new maps, edit existing maps, and transfer maps between computer systems that are not binary-compatible. Since SSF files are ASCII files, they may be created or changed using a text editor. SSF files use the same format as NCAR sequential map files.
The Named (NAM) format is similar to the SSF format. It too is an ASCII format. It includes names (such as country, or river names) associated with a group of map records.
In all forms of the GEMPAK map files, latitude and longitude are given in degrees. West longitude is negative. All longitudes are between -180 and +180 degrees. No map segments cross the International Date Line.
A SSF file contains map segments, each of which is a series of latitude/longitude points to be connected by straight lines when the map is drawn. Each segment contains the following information:
NP2 INTEGER Number of points (np) * 2
RMXLT REAL Maximum latitude in segment
RMNLT REAL Minimum latitude in segment
RMXLN REAL Maximum longitude in segment
RMNLN REAL Minimum longitude in segment
PTS (NP2) REAL LAT1, LON1, ..., LATnp, LONnp
These segments are written and read with the following FORTRAN format statement:
FORMAT ( I4, 14X, 6F9.3, 8X, / ( 8F9.3, 8X ) )
A segment may include at most 3000 points.
A GSF file is a direct access file with a record size of 128 words. The data is written in blocks of two records, or 256 words. The first block is the file header, followed by data blocks. Map segments are packed into the data blocks; no map segment spans blocks.
The file header has 181 INTEGER*2 words containing the following
information:
NMBLK INTEGER*2 Total number of blocks
LATPTR1 INTEGER*2 Latitude pointer for 90 degrees
LATPTR2 INTEGER*2 Latitude pointer for 89 degrees
:
:
LATPTR180 INTEGER*2 Latitude pointer for -89 degrees
The GSF files are sorted by maximum latitude. The LATPTR array contains the first block to use given the maximum latitude in a plot.
These INTEGER*2 values may be changed to INTEGER*4 if required. The changes must be made in the conversion programs GSFSSF and SSFGSF, described below, as well as in GDRMAP, the GEMPLT subroutine that draws maps.
The data blocks contain only 4-byte integer or real words. Each data block contains the following information:
NMSEG INTEGER Number of segments
----- ------ Unused
NP1 INTEGER Number of points in segment 1
RMNLT1 REAL Minimum latitude
RMNLN1 REAL Minimum longitude
RMXLT1 REAL Maximum latitude
RMXLN1 REAL Maximum longitude
----- ------ Unused
RLAT1 (1) REAL Latitude of point 1
RLON1 (1) REAL Longitude of point 1
:
:
RLAT1 (np1) REAL Latitude of point np1
RLON1 (np1) REAL Longitude of point np1
NP2 INTEGER Number of points in segment 2
RMNLT1 REAL Minimum latitude
:
:
Map files are named by concatenating two-letter acronyms for the resolution, map boundaries, and area with a three-letter origin code. This is described in the discussion of the GEMPAK variable $MAPFIL; see the NAWIPS User's Guide.
Some GSF format map file names end with a .gsf suffix. The suffix indicates the origin of the map; a .gsf suffix indicates that a map originated at the Goddard Space Flight Center (GSFC). Note that ""GSF" and .gsf have different meanings.
The program GSFSSF is used to convert any GSF file to a SSF file. The user is prompted for the existing GSF file name and a name for the SSF file to be created.
gsfssf -o output.ssf input.gsf
The program SSFGSF is used to convert a SSF file to a GSF file. The user is prompted for the existing SSF file name and a name for the GSF file to be created.
ssfgsf [options] -o output input
-w subset Define subset area in the form of
MINLAT;MINLON;MAXLAT;MAXLON
If no subset is provided, the whole SSF file will be converted.
-h Display this help section.
ssfgsf -o hiosuo.gsf hiosuo.ssf
the program converts the whole SSF file into GSF file.
ssfgsf -w 19.00;-119.00;47.00;-56.00 -o hiosuo.gsf hiosuo.ssf
the program converts those points within the given bounds.
The user can also enter a latitude/longitude window to subset the data. Any point outside the window is eliminated from the output file. However, no clipping is done. If a partial or no subset is desired, "missing" values (-9999) are used for the latitude and longitude bounds.
The minimum and maximum latitude and longitude stored with each segment is recomputed internally, so the values in the SSF file need not be correct. The intermediate files, DAFIL.INT and SEQFIL.INT, are used by the program and may be deleted after the conversion.
Utility NAMCHECK is used to check the binary format (GSF) map files for any records that cross the International Date Line. GEMPAK map files have no segments that cross this boundary.
GEMPAK maps include a subset of the CIA map database. Geographical names were added to each record in this database at the Goddard Space Flight Center by Joseph Fulson-Woytek.
The result is a file named CIA.NAM and it is located in directory $GEMMAPS/nam. Several of the GSF maps in $GEMMAPS were derived from this CIA source.
File DOC.NAM, located in $GEMMAPS/nam, includes more information about the CIA.NAM file.
File REC.NAM, also located in $GEMMAPS/nam, is a list of records (an index) for the map elements in CIA.NAM.
Utility programs NAMSSF, NAMPLT, NAMUPD, and SSFNAM are used in conjunction with CIA.NAM.
NAMSSF converts a subset of CIA.NAM into a SSF file. The program prompts for the following inputs:
AREA Any geographic name or a range of records specified by the start and stop records, separated by a dash ("-").
REGION A string listing the regions to be used (a blank defaults to all) where the valid regions are:
C - Continental borders
I - Islands
L - Lakes
S - State borders
N - National borders
R - Rivers
T - Tectonic plates
M - Miscellaneous
NAMSSF Example
A map containing only Mexico borders is created as follows.
AREA Mexico
REGION CN
In this example, adding "S" to the region specification would include "New Mexico;" adding "I" would include "Gulf of Mexico Islands."
NAMPLT plots individual records. It is used for debugging a map file. Before running NAMPLT, the GEMPAK program GPMAP is executed to select a GAREA and a plot device. NAMPLT is executed second. As it runs, it plots each record, lists it, and waits for a <CR> before proceeding on to the next record.
SSFNAM converts a SSF file into a NAME file.
NAMUPD updates records in the CIA.NAM database. It creates new files containing the names of all the records. The new files are CIA.NAM_NEW and REC.NAM_NEW. File REC.NAM_TMP is a temporary file that may be deleted.
If anyone makes modifications to CIA.NAM, the GEMPAK developers will very much appreciate a copy of the changes so that all of the GEMPAK map files that are based on CIA.NAM can be updated. This includes all those map files named with the .cia suffix.