PROBLEM
This problem is addressed in ArcGIS 8.1 Service Pack 1. See the Related Information section to download this pack.Use the drawing tools to draw a graphic over a portion of a raster layer that is symbolized using a string field. The histogram produced by clicking the Histogram tool of ArcGIS Spatial Analyst gives incorrect results.
This is a known issue with ArcGIS 8.1.
Article ID:000003339
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