ArcMap automatically projects data layers to the data frame's coordinate system. This is called 'project-on-the-fly'. However, if the Geographic Coordinate Systems (GCS) are not the same, a geographic (datum) transformation must be applied by the user to the ArcMap data frame. This is in addition to the operation performed with project-on-the-fly, which temporarily converts coordinate values from the projection definition of the data to the coordinate system applied to the ArcMap data frame.
The geographic (datum) transformation cannot be automatically applied because many geographic coordinate systems and transformations exist for a given area. Geographic transformations use different methods, with different accuracies, and are defined for particular areas. This document discusses how to manually apply a geographic transformation to the ArcMap data frame.
Warning: Do not change the value in the 'Into' box in this dialog.
Note: It is okay if the transformation name goes in the opposite direction. ArcMap applies the transformation in either direction with equal accuracy.
Warning: Only one transformation can be selected in this dialog.