Instructions for Importing an ASCII Grid into Arcview GIS Using Spatial Analyst

These instructions are specifically designed to help users import raster PRISM grids into Arcview GIS.

1. Download the desired ASCII raster grid from the Oregon Climate Service PRISM web page (www.ocs.orst.edu/prism/prism_new.html) IMPORTANT: Be sure you download the ASCII raster grid with the ARC/INFO header in BINARY mode (these files are compressed using gzip). These files have the following naming convention: <state,region>_<variable>.<month>. (e.g. us_ppt.01). The letter "g" after the month indicates it is a GRASS ASCII file. Also NOTE! The file numbers refer the the months, for instance us_ppt.03.gz is mean monthly precipitation for MARCH; 14 refers the the annual file.
   
2. Once the raster file is on a local drive, use gunzip to uncompress the file (<filename>.gz). Download a shareware utility - if you do not already have one - to gunzip the file.
   
3. Start Arcview GIS and be sure the Spatial Analyst extension is turned on. The do this, select File and Extensions... and check Spatial Analyst.
   
4. Open existing or start a new project. Open and make a View active.
   
5. Now import the gridded ASCII file you gunzipped, by selecting Fileand thenImport Grids.... The Import File Type should be set to ASCII.
   
6. In the Import ASCII Files dialog box, you must change the "List Files of Type" to All Files (*.*). Select the unzipped ASCII file.
7. At this point you must provide Arcview GIS with an output file name (the default is imgrd1) for the imported grid . Be sure you are saving in the correct directory and then type in the filename.
   
8. At the dialog box "Import Grids: ..." Cell values as integers? Select "yes."
   
9. Now go to a View, and add a theme. In the Add Theme dialog box, change type of theme from Featured Data Source to Gridded Data Source. Then simply select the imported grid that you saved in step 7.

   NOTE: A problem has been detected with the use of the PRISM
    precipitation grids (units=mm*100) with ArcView's Spatial Analyst
    [NT/win95 Arcview version 3.0a and spatial analyst v1.1.  Unix ArcView
    v3.0b and Spatial Analyst 1.0a.] The grid, once imported, does not
    display properly.  An easy fix has been found to allow ArcView users to
    use these grids.  Using ArcView's map calculator, divide the
    precipitation grid value by 100. The map will now display correctly and
    units will be in mm.

10. Depending on the grid, the nodata_value is -999 or -9999.
    
11. When the new theme is turned on, the view will show the latitude-longitude projected gridded data.
    
12. To conserve disk space, you can now delete the original gunzipped file.
    

For additional Arcview GIS or Spatial Analyst HELP, please contact ESRI (www.esri.com).

For information on the gridded data (i.e. units, projection, etc.) please refer to the accompanying metadata and README files.

PRISM Group, Oregon State Univeristy
12 July 1999

Copyright © 1999 by the PRISM Group, Oregon State University