Understanding GPS and Datums

A datum is a frame of reference for mapping. Because the earth isn’t flat, geographic coordinate systems use ellipsoids (think of a sphere that’s not perfectly spherical, much like the shape of the Earth) to calculate positions on our third planet from the sun. A datum is the position of the ellipsoid relative to the center of the earth.
Unless you’re a cartographer or geographer, that probably hurts just thinking about it. Sparing you a long and detailed technical description, here are the two important things that you need to know:

• All maps have a datum. Hundreds of different datums are in use. Most good maps used for navigation — and highway maps don’t count —usually state which datum was used in making the map.
• You can set what datum your GPS receiver uses. The default datum for GPS receivers is WGS 84, more formally known as the World Geodetic System 1984. WGS 84 was adopted as a world standard and is derived from a datum called the North American Datum of 1983. Most USGS topographic maps that you use for hiking are based on an earlier datum called the North American Datum of 1927, or NAD 27. This datum is divided into different geographic areas, so if you’re in the United States — at least in the lower 48 states — use a version of NAD 27 that mentions the continental U.S.

So why is all this datum stuff so important? In the United States, if a position is saved in a GPS receiver by using the WGS 84 datum and the same coordinates are plotted on a map that uses the NAD 27 datum, the location can be off as much as 200 meters. That’s more than a couple of football fields off. The latitude and longitude coordinates will be identical, but the location is going to end up in two different spots.
The moral of the story is to make sure that the datums on your GPS receiver and maps are the same. Or, if you’re with a group of people using GPS receivers, make sure that all your datums match.
Not having the map datum match the GPS receiver datum is one of the biggest errors that new users of GPS receivers make. I can’t emphasize this point strongly enough: Make sure that the two match!