When a raster with a spatial reference, such as UTM13N NAD27, is projected on-the-fly to another datum, such as UTM13N NAD83, the two images may not line up properly when displayed in ArcMap.
Datum transformation is not supported with rasters. This is a known issue with ArcGIS 8.1.
This issue will be resolved in ArcGIS Version 8.1.2.
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