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It is necessary to specify a geographic (datum) transformation when using the ArcToolbox Project Wizard (ArcGIS version 8.x) or Project Tool (ArcGIS version 9.x) to project shapefiles or geodatabase feature classes between different geographic coordinate systems (datums).
A user must also select an appropriate Geographic (datum) transformation in ArcMap when displaying data that are projected on different datums. For more information on selecting an appropriate Geographic transformation, refer to ArcGIS Help: Choosing an appropriate transformation.
Click a link in the Related Information section to access a list of the geographic (datum) transformation methods available in ArcGIS, and the geographic areas for which these transformations are appropriate for the version of ArcGIS Desktop used.
Note: Datum transformations work in either direction. For example, the transformation listed as NAD_1927_to_NAD_1983_NADCON transforms from NAD 1983 to NAD 1927, as well as from NAD 1927 to NAD 1983.