Instructions for Importing an ASCII Grid
into Arcview GIS Using ArcToolbox or Spatial Analyst(older software)
First, download the desired ASCII raster grid from the PRISM Climate
Group web page (http://prism.oregonstate.edu).
File name suffix referes to the month number (e.g. 01 = Jan, ... 12 = Dec, 13
or 14 = Annual). After download, you MUST rename the file with the suffix txt
or asc. ESRI software will not read the file name in any other convention.
For help in downloading the ASCII grids, you might also look at the follwing
web page: download.html
- From ArcToolbox, select Conversion Tools - To Raster - ASCII to Raster
- Select the file icon in the far right of the Input ASCII Raster File file
- A dialog box will open. Select the ASCII file for import.
- Select the Output Raster.
- Select Output Type as Integer or Float.
- Select the OK button and the data will start importing.
- 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.
- Start Arcview GIS and be sure the Spatial Analyst extension
is turned on. The do this, select File and Extensions... and check Spatial
- Open existing or start a new project. Open and make a View active.
- Now import the gridded ASCII file you gunzipped, by selecting Fileand
thenImport Grids.... The Import File Type should be set to ASCII.
- In the Import ASCII Files dialog box, you must change the "List Files
of Type" to All Files (*.*). Select the unzipped ASCII file.
- 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.
- At the dialog box "Import Grids: ..." Cell values as integers?
- 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.
- Depending on the grid, the nodata_value is -999 or -9999.
- When the new theme is turned on, the view will show the latitude-longitude
projected gridded data.
- To conserve disk space, you can now delete the original gunzipped file.
additional Arcview GIS or Spatial Analyst HELP, please contact ESRI (www.esri.com).
information on the gridded data (i.e. units, projection, etc.) please refer to
the accompanying metadata and README files.
PRISM Group, Oregon State Univeristy
3 May 2010
Copyright © 199X, 20XX by the PRISM Climate Group, Oregon State University