Dear Capt. Paul:

I like to find potential spots on Google Earth. I have heard that the coordinates from Google Earth can be off a good bit from what a GPS unit says if you are not using the right datum.

Can you tell me what datum I should when finding spots on Google Earth?

Thanks,Dan Redwood

Capt. Paul’s response:

Unfortunately, the Google Earth program does not state what datum they’re using on their displayed aerial photo maps. It is probably WGS 84, but I am not sure.

I plotted a position, my front mailbox, with the Google Earth program and then with my Maptech Terrain Navigator Pro and then with my GPS running.

The Maptech program uses Ortho (Corrected) Aerial photos, and is usually very, very accurate.

The Maptech and the GPS unit were set to WGS 84 datum, and they indicated a position that that was about 13.8 feet difference. One fixed the location as NXX 39.137 x WXX 49.834, and the other was NXX 39.138 x WXX 49.835.

Using the Google Earth program, the specified location was about 72 feet away from the average of the other two fixes. I assumed by that that the Google Earth program was using WGS 84 datum, since a setting of NAD 27 datum would be nearly double the indicated position fix error.

I again assumed that the difference may have come from the angle that the Google program allows the user to view the position. The more oblique the angle, that is the more it is set to the eye-level view, the harder it is to pinpoint a specific point with accuracy.

I would certainly use the program to narrow down a specific location, but I would refine the position by using a GPS unit that has the WAAS differential feature to provide the most accurate fix. Such a unit should provide an accuracy reading to within 3 meters (9.8 feet).