A point feature layer or event theme must be loaded into ArcMap in order to use this
tool. Select the layer you wish to process from the drop-down box. The tool will search the attribute table for
common coordinate field names. If any are found, the names of those fields will appear in the X and Y field text
boxes and the user will be given the option of updating those fields (the "update fields" check box). If
new fields are desired, simply uncheck the box and type in the new field names in the X and Y field text boxes.
The projection of the coordinates added to the attribute table can either match the dataset, or the
map window. If no projection for the dataset is defined you will see "Unknown" listed and no re-projection is possible.
To define the projection of the source data use the Define Projection wizard in ArcToolbox (note that the
layer cannot be open in ArcMap when you do this). To change the coordinate system of the map window, right
click on the map and go to properties. Select the Corodinate System tab, and change the coordinate system to
whatever you choose. What you start the AddXY tool, the coordinate system of the dataframe is displayed near the bottom
of the form. Note that some re-projections between coordinate systems are more complex than others. For instance,
projecting between Geographic Coordinates (WGS84) and UTM (WGS84) is fairly straightforward, but switching between
some datums (e.g. NAD27 and NAD83) is more complex and may raise errors. If you are unable to add re-projected coordinates
using this tool, it is recommended that you use the reprojection tools in ArcToolbox to reproject your dataset,
and then use the AddXY tool.