Having trouble displaying a shapefile in 9.2 and/or having it line up with other geospatial data when it used to work just fine in version 9.1?
Up until version 9.2, a loosely defined coordinate system known as GCS_Assumed_Geographic_1 was "assigned" to a shapefile when ArcGIS "saw" decimal degree coordinates for a shapefile, even though there was no associated .prj file to specify the exact parameters (datum) of the coordinate system.
This GCS_Assumed_Geographic_1 coordinate system has been removed from version 9.2; thus anything without a .prj file will now be assigned "Unknown".
ArcMap 9.2 will not be able to project them on the fly to match other layers in the map, and you will get a warning that the spatial reference has not been defined. Data that used to align correctly in ArcGIS version 9.1 or earlier will no longer line up in version 9.2.
It is now more important than ever to use the Define Projection tool (or other means) to create a .prj file with the correct coordinate system parameters for all of your geospatial data files.
For details on why ESRI removed the GCS_Assumed_Geographic_1 coordinate system, refer to this technical article.
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