Pacific Northwest Climate Maps

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Map Data Format

The data files for these maps are in ESRI, Inc.'s ASCII Grid format. The header for each file is:

ncols         128
nrows         96
xllcorner     -125
yllcorner     41
cellsize      0.125
NODATA_value  -9999

We settled on this format because it (1) is widely supported and easy to convert with a small script if necessary, (2) is an ASCII format so it is both human readable and hardware independent; and (3) is easy to import and export from CIG’s GIS software (ArcGIS).

To import ASCII Grids into common GIS packages:

• ArcGIS: Use ArcTools's Import ASCII to GRID function
• ArcView: use the import ASCII Grid function (May need Spatial Analyst)
• GRASS: Use the ' r.in.arc ' function.

For those software packages that do not natively support ESRI ASCII Grids, the description of the data format follows from the ArcInfo Workstation 9.1 help file is at the end of this webpage.

Projection

These ASCIIGRID files are all in geographic coordinates. Use the following projection information with PROJECTDEFINE in ArcInfo:

ESRI ASCII Grid Format

The ASCII file must consist of header information containing a set of keywords, followed by cell values in row-major order. The file format is:

<NCOLS xxx>
<NROWS xxx>
<XLLCENTER xxx | XLLCORNER xxx>
<YLLCENTER xxx | YLLCORNER xxx>
<CELLSIZE xxx>
{NODATA_VALUE xxx}
row 1
row 2
.
.
.
row n

where xxx is a number, and the keyword nodata_value is optional and defaults to -9999. Row 1 of the data is at the top of the grid, row 2 is just under row 1 and so on.

For example:

ncols 480
nrows 450
xllcorner 378923
yllcorner 4072345
cellsize 30
nodata_value -32768
43 2 45 7 3 56 2 5 23 65 34 6 32 54 57 34 2 2 54 6
35 45 65 34 2 6 78 4 2 6 89 3 2 7 45 23 5 8 4 1 62 ...

The nodata_value is the value in the ASCII file to be assigned to those cells whose true value is unknown. In the grid they will be assigned the keyword NODATA.

Cell values should be delimited by spaces. No carriage returns are necessary at the end of each row in the grid. The number of columns in the header is used to determine when a new row begins.

The number of cell values must be equal to the number of rows times the number of columns, or an error will be returned.