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8.0 Grids and Grid Files - Basic Concepts

Grids

A grid is a lattice of regularly spaced data points superimposed on a projection of the earth. Grids can store numerical model data and serve as an interface between observational data stored in MD files and analyzed data displayed as contours on a frame. To contour observational data, compute derived parameters, and display manually digitized radar (MDR) data, etc., the data must first be interpolated from the observing locations to a uniform latitude, longitude lattice. McIDAS stores this geographic lattice as a grid.

Grid Files

Grids are stored in grid files. Grid files use the naming convention GRnnnnnn where nnnnnn is the six-digit file number. If the file number is less than six digits, I and D replace the first two digits and zeros precede the number. For example, GR111111 is the name of the file for grid 111111, GRI90112 is the name for grid file 90112, and GRID0013 is the name for grid file 13. In ADDE, sequences of GRID files are grouped together in a dataset.

Grid File Format

Each grid file contains a grid file directory and up to 10,000 grids. Each grid contains a grid header and grid data, as shown below. A sample grid dataset directory and a listing of grid header information are also shown below.


GRDLIST BLIZZARD/GRIDS NUM=ALL FORM=FILE
DATASET NAME: BLIZZARD/GRIDS
 Dataset Position  Creation Date Max Grids         Directory Title
 ----------------  ------------- --------- -------------------------------
        1             1993070        159   12Z 93070 NMC NGM TRAINING GRID
        2             1993071        159   00Z 93071 NMC NGM TRAINING GRID
        3             1993071        159   12Z 93071 NMC NGM TRAINING GRID
        4             1993072        159   00Z 93072 NMC NGM TRAINING GRID
        5             1993072        159   12Z 93072 NMC NGM TRAINING GRID





GRDLIST BLIZZARD/GRIDS.1 GRID=1 FORM=ALL
Dataset position 1      Directory Title= 12Z 93070 NMC NGM TRAINING GRID
PARAM  LEVEL      DAY        TIME    SRC FHOUR     FDAY       FTIME   GRID  PRO
----- ------- ------------ -------- ---- ----- ------------ -------- ------ ----
Z     1000 MB 11 MAR 93070 12:00:00  ROI     0 11 MAR 93070 12:00:00      1 MERC
Total pts= 1188  Num rows=  33  Num columns=  36    received:      0 000000Z
No GRIB number information 
Units of gridded variable are GPM  Scale of variable is:   2
Mercator Projection
Min Lat=  20.00  Max Lat=  60.00  Min Lon=  52.50  Max Lon= 140.00
Latitude Increment= 1.2500  Longitude Increment= 2.5000
Number of grids listed = 1

Manipulating Grids and Grid Files

The GRDLIST and GRDCOPY commands manipulate grids in a grid file. GRDLIST can list grids in a grid file, while GRDCOPY can copy, move, and perform mathematical operations on grids. These operations include adding or subtracting two grids, filling a grid with a constant value, or redefining all values within a grid that are less than a specified number to be a new number (e.g. all values less than zero are set to zero). If you have u and v component grids, you can create a vorticity or divergence grid, or use the grid components to advect a parameter in another grid.

The GRDINFO command provides information about grids. You can use it to list information about the grid header and statistics on the grid data, or list data points.

The GRDLIST and GRDCOPY commands can also manipulate grid files. With these commands you can create, list, or copy a grid file.

XCD Decoded Real-Time Grid Data

Real-time grids from NCEP (National Center for Environmental Prediction) forecast models are included in the Unidata IDD HRS and CONDUIT datastreams. You can decode these data using the McIDAS-XCD decoders. The following ADDE datasets are set up to access the real-time gridded data:


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