This problem is addressed in ArcGIS 8.1 Service Pack 1. See the Related Information section to download this pack.
If a shapefile is used as a mask for an analysis operation, an error, "The cell size value is either invalid or not defined", is returned when using the Raster Calculator.
This is a known issue with ArcGIS 8.1.
Solution or Workaround
This is fixed in ArcGIS 8.1 Service Pack 1.
Alternatively, specify a cell size in the Cell Size tab of the Options dialog before performing an operation in the Raster Calculator using a mask.
- Click the ArcGIS Spatial Analyst dropdown arrow and click Options.
- Click the Cell Size tab and specify a cell size for the analysis results.
- Perform the analysis using the Raster Calculator.