** This error has been fixed at ArcGIS Desktop 9.1 **
When a string field is used in the Spatial Analyst Reclassify tool and set as the Old Values column, a string field is added to the raster output from Reclassify. The strings in the rows of this output string field are created by grouping the characters from the list of attributes in the Old Values column, that is, all the attributes mapped to the same new value.
Since there is a limit of 256 characters on the length of a string field, reclassify fails to run if this limit is reached when processing and displays the following error message:
"("esriGeoAnalyst.GridEngine") Field not found. Invalid field index obtained."
The limit on the length of a string field of 256 characters has been reached.
For example, with the following remap table:
Old value New value
"Value1" = 6
"Value2" = 6
"Valueabc" = 8
"Value4" = 6
"Value1"+"Value2"+"Valueabc"+"Value4" = 6 + 6 + 8 + 6 = 26
([new_stringField] = Left([stringField], 3)
The number 3 represents the number of characters to take, starting from the left. Depending on how many attributes are in the string field, specify a lower number or use a higher number. Ultimately, the string field in the output raster must be less than 256 characters in length.