McIDAS Programmer's Manual
Version 2003

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GRIDnnnn

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

A grid has two components:

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, degrees*10000

36

maximum longitude of the grid, degrees*10000

37

minimum latitude of the grid, degrees*10000

38

minimum longitude of the grid, degrees*10000

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, degrees*10000

39 - 40

standard latitudes, degrees*10000; set these two equal for polar stereographic

Remaining header words if grid type = 3 (equidistant)

Header Word Description

35

latitude of (1,1), degrees*10000

36

longitude of (1,1), degrees*10000

37

clockwise rotation of column 1 relative to north, degrees*10000

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, degrees*10000

39

standard latitude, degrees*10000

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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