Projecting

When we have data in one spatial reference (either a GCS or PCS) and we need to convert it to another, we refer to this as "projecting" the data.

If the datums are the same in both the spatial references, then we can just project the data to geographic coordinates (GCS) and then project them to the new projection.

 

If the datums are different, however, we need to add another step to change the data from one datum to another. This is called a "Geographic transform" or "datum transformation".

Esri's Datum Transforms

Selecting the Datum Transformation

ArcMap

When you "Project" data in ArcMap, a green dot will appear next to the "Geographic Transformation" popup box. This box should really be called Datum Transforms because that is all it contains and there are many more geographic transformations (including projecting). Sometimes, ArcMap will select a transformation for you which is OK if you are not worried about a few meters of error (I have seen up to 16 meters). Otherwise, you'll need to scroll through this list to find the transformation that is appropriate for your data. If there is more than one, you can use the HSU Data Hub to select the best transform for your area.

Note: Esri used to have a page containing the areas for each Datum Transformation but the link is no longer valid, extra credit for someone who finds it!

FWTools

For transforming data from one North American Datum (NAD27, NAD83), to another, you can use a single "NADCON" transformation. For most other transforms, we need to transform the data to WGS84 and then transform it to the new datum.

HSU Data Hub EPSG Search Tool

For selecting datum transformations for tools other than ArcMap (like FWTools), you can use the EPSG database. An easy way to access the EPSG data base is using the HSU Data Hub.

1. After reaching the data hub, select "EPSG"
2. Click on "Coordinate Reference Systems"
3. Enter search criteria to find the source GCS you are interested in. You may need to try some different approaches like entering NAD27 and then entering "NAD 27" if the first search does not show your CRS. Once you find your CRS, it is good to write down it's EPSG number as these are used more and more frequently to refer to CRSes (ArcMap now includes them in many of their PRJ files) and it will help you find your transformation.
4. Repeat the step above to find your destination GCS.
5. Click on "Coordinate Reference Operations" and enter your source EPSG code and see if an appropriate operation appears, if not, try using the source as the target. Continue until you find the best transformation.

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