Not long ago we received a question via Ask a Cartographer that our map projection experts frequently get; it goes something like this:  "When projecting data to WGS84, for example, which transformation is the best to use?"

There are two contexts for this scenario in ArcGIS. The first is as described above, namely, you need to project data and are potentially confronted by a lot of transformation methods and don't know which to choose.  The second context is a bit more complex, which is that you've added data from several sources to your map and you get a somewhat verbose warning (shown at right) that is, if you are like us, all too easy too ignore since it looks complicated.

The response to both scenarios is: you will need to test with your data to determine which transformation works best. That means learning how to handle the second context will give you the what you need to handle the first context.

To test your data, start with a new empty map, then set the data frame's coordinte system to the coordinate system you want to view your data with or project your data to.  

TIP:  ArcMap will automatically set the coordinate system to that of the first dataset you add to a new empty data frame. Just removing all the layers from an existing data frame won't work; ArcMap will still consider it "dirty" and won't change it's coordinate system when you add a new layer.

To facilitate explaining how to do this, let's assume that we've set our data frame to use North American Datum,1983 (NAD83) as it's coordinate system, and that we'll be adding a dataset that uses WGS84 and another that uses a longitude/latitude NAD83.   

This gets us to the meat of the problem, you will need to select which transformations to use in order to get the data to align. To do that you need to click the Transformations button, either on the Geographic Coordinate System Warning dialog (shown at the upper right) or in the Data Frame Properties dialog's Coordinate System tab. That will show the Geographic Coordinate System Transformation dialog.

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To specify the transformation, select any of the geographic coordinate systems (GCSs) in the top list. The "Into" dropdown combo box should already be set to the GCS your data frame is using.  Then you can set the "Using" method.  Depending on the combination of GCSs there may be several that make sense, or none at all. ArcMap is helping you at this point, by showing you only the methods that are likely to pertain to the combination of GCSs you are using. 

TIP:  The methods' names are expressed in a GCS to GCS style; it does not matter which comes first, just find one that includes the pair of GCS names you are working with. 

Once you have set the transformations, verify by clicking on each of the GCSs in the top list and you should see a transformation appear in the "Using" box, except for the case where the selected GCS matches the data frame's GCS, which should come up as as "<none>". 

TIP:  There is one case where ArcMap will do something automatically, this is when there is a pairing of NAD27 and NAD83 data, in which case ArcMap will automatically specify the NADCON transformation.  This is the only combination of coordinate systems for which ArcMap will automatically apply a transformation.

Once you've chosen all the needed transformation methods, explore your map and see how well the transformed data conforms to your other data (presumably already in the GCS you would like to project to).  If it's acceptable, then you'll know what transformation method to use when you project the data.

In general, this is a tedious process.  However, it is far less tedious than finding and fixing errors late in your workflow that are essentially errors of assumption about whether to transform or which transformation method to use.  Ironically, in many cases we've found that the ‘errors' in our data did not have much to do with the transformation method itself.  Rather, they had to do with differences in data capture and production.  When you use datasets, created by different organizations or at different times, to match perfectly is pretty hard to do and doesn't happen very often.

One last item to note is the popup help (click the "?" button on the Data Frame Properties dialog and then click the Transformations button) is not accurate in that ArcMap doesn't automatically select a transformation when you add data unless the NADCON transformation method can be used.

To learn more about how ArcGIS projects and transforms data check out the Overview of Projections and Transformations toolset help topic, which explains why you need to be aware of transformations and explains which tools can help you get your data properly transformed and projected.