Projecting Data between NAD27 into NAD83 Coordinate Systems in Alaska

Many Boroughs, the State of Alaska, as well as some federal agencies maintain their datasets in coordinate systems based upon the North American Datum of 1927 (NAD27).  NRCS has standardized their applications and datasets upon the more modern NAD83 datum and the UTM coordinate system.

Arcmap will project data from differing coordinate systems “on the fly” so that everything will line up properly.  On-the-fly projection works well in situations where all datasets are referenced to the same datum.  Unfortunately when projecting from the NAD27 datum into NAD83, Arcmap uses a default transformation designed for the lower 48. If this default behavior is used in Alaska, datasets based upon NAD27 will be incorrectly registered when overlain with datasets using NAD83 datums. Projection errors will typically be on the order of by several hundred meters. (See example below right).

1). To properly project data from NAD27 to NAD83 in Alaska you must specify an appropriate
transformation method. To do this, right click on the data frame in the table of contents and
select properties. (see above)

2). Under the Data Frame Properties popup box, Click on the tab for coordinate systems.

3). Then click on the Transformations button.

4). The transformation dialog box gives you the opportunity to specify an Alaska based method for projecting the data. – Here you see the default transformation uses the NADCON method for the Continental US. – You can change the method in the dropdown list under Using.

You may have a situation where other transformation methods are not listed in the dropdown selection box. This will be the case if all the data is referenced in the same datum or you may need to select different datums in the Convert From and Convert Into dialogs.

5). Choose the NAD_1927_To NAD 1983_Alaska as the transformation method
under the Using dropdown list.

Click OK. 

6). Finally Click OK again under the Coordinate Systems tab.

7). Your data should now be properly aligned.

The noticeable shift between the NAD27 Parcels layer and the Parcel layer in the NAD83 coordinate system is gone.

In the future when you see shifts of approximately 200 meters you should first suspect a transformation issue between NAD27 data and the NAD83 data.

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