GRIDnnnn

Grid files, where nnnn is a user-defined number.

A grid has two components:

64-word header describing the grid projection, grid directory, gridded variable, level parameter, time of the data, etc.
Actual grid data, which is an n- by m-word matrix

A grid file is a binary file, which may contain a user-defined maximum number of grids. By default, a grid file is created with the ability to store 159 grids unless otherwise specified.

Grid file numbers can be between 1 and 999999. If a grid file number is five or six digits, the file name begins with only GRI or GR. For example, grid file number 12345 has the file name GRI12345, but grid file number 123456 has the file name GR123456.

Grid files do not have to follow this standard naming convention. The file masking option of DSSERVE may be used to access a data file of any name through the ADDE.

The word allocation for grid files is divided into several sections below. The grid file directory is described first, followed by the grid header. The first 33 words and words 40 through 64 of the grid header are the same for all grid types; however, words 34 to 39 are specific to a particular grid type.

For grid file Applications Program Interfaces (APIs), refer to the API functions list at the end of this section.

Grid file directory

Word	Description
0 - 7	32 characters of label information
8	project number used to create the grid file
9	date the file was created, yyyddd
10	maximum number of grids (n) in the grid file
10+1	word offset, from the beginning of the grid file, where grid 1 starts; if the offset is -1, no grids exist
...	...
10+n	word offset for grid n
10+ n+1	next available address to start writing the next grid

Grid header

Each grid header contains 64 words. The offset of the first word in the header is defined by the word offset in Words 10 +1 through 10 + n in the grid file, where n is the grid number.

Header Word	Description
1	total size; rows * columns (not to exceed the value of MAXGRIDPT in gridparm.inc)
2	number of rows
3	number of columns
4	Julian date of the data, ccyyddd
5	time of the data, hhmmss
6	forecast time for the grid, if applicable
7	name of the gridded variable, four character ASCII
8	scale of the gridded variable, specified as a power of 10
9	units of the gridded variable, four character ASCII
10	value of the vertical level 1013 = 'MSL' 999 = ' ' 0 = 'TRO' 1001 = 'SFC' (Otherwise, it is displayed as entered.)
11	scale of the vertical level
12	unit of the vertical level
13	gridded variable type: 1 = time difference 2 = time average 4 = level difference 8 = level average (or any sum of 1, 2, 4 and 8)
14	used if the grid parameter is a time difference or time average, hhmmss
15	used if the grid parameter is a level difference or level average; values are the same as Word 9
16 - 32	reserved
33	grid origin; identifies the type of program that generated the grid data
34	grid projection type: 1 = pseudo-Mercator 2 = polar stereographic or Lambert conformal 3 = equidistant 4 = pseudo-Mercator (more general) 5 = no navigation 6 = tangent cone
35 - 40	varies, depending on the grid type; see the GRIDnnnn data file in Chapter 6 for more information
41 - 52	reserved; filled only if the grid was created by the McIDAS-XCD GRIB decoder
49	geographic grib number
50	parameter grib number
51	model grib number
52	level grib number
53 - 64	grid description

Remaining header words if grid type = 1 or 4 (pseudo-Mercator)

Header Word	Description
35	maximum latitude of the grid, degrees10000*
36	maximum longitude of the grid, degrees10000*
37	minimum latitude of the grid, degrees10000*
38	minimum longitude of the grid, degrees10000*
If TYPE=1:
39	increment between grid points; same in x/y directions
40	reserved
If TYPE=4:
39	increment between the grid points (latitude)
40	increment between the grid points (longitude)

Remaining header words if grid type = 2 (polar stereographic or Lambert conformal)

Header Word	Description
35	row number of the North Pole*10000
36	column number of the North Pole*10000
37	column spacing at standard latitude, meters
38	longitude parallel to columns, degrees10000*
39 - 40	standard latitudes, degrees10000*; set these two equal for polar stereographic

Remaining header words if grid type = 3 (equidistant)

Header Word	Description
35	latitude of (1,1), degrees10000*
36	longitude of (1,1), degrees10000*
37	clockwise rotation of column 1 relative to north, degrees10000*
38	column spacing, in meters
39	row spacing, in meters

Remaining header words if grid type = 5 (no navigation)

Header Word	Description
35 - 40	reserved

Remaining header words if grid type = 6 (tangent cone)

Header Word	Description
35	row number of the North Pole*10000
36	column number of the North Pole*10000
37	column spacing at standard latitude, meters
38	longitude parallel to columns, degrees10000*
39	standard latitude, degrees10000*
40	reserved

Reserved header words if grid was created by the McIDAS-XCD GRIB decoder

For more information, refer to the National Centers for Environmental Prediction Office Note 388: GRIB, Edition 1.

Header Word	Description
47	date grid was filed, yyyddd
48	time grid was filed, hhmmss
49	original geographic ID; GRIB projection number (PDS octet 7)
50	original parameter ID; GRIB parameter number (PDS octet 9)
51	original process ID or model number (PDS octet 6)
52	original level type (PDS octet 10)

Grid data

Word	Description
offset + 64	beginning of the grid data
...	...
offset + 64 + (rows*columns) - 1	end of the grid data

API functions

Function	Description
mcgdir	opens a connection to read a grid file directory
mcgdrd	reads a grid file directory
mcgfdrd	reads a grid file header
mcgget	opens a connection to read a grid
mcgridf	reads a grid in Fortran (column-major) order
mcgridc	reads a grid in C (row-major) order
mcgput	opens a connection to write a grid
mcgoutf	writes a grid in Fortran (column-major) order
mcgoutc	writes a grid in C (row-major) order
igquit	deletes a grid file
m0gsort	gets grid selection parameters from the command line

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