Adjusting GPS receivers for the correct datum

By Susan Rhea

Over at the Denver Federal Center, near Building 810 where USGS topo maps are sold, is a wonderful display. In the concrete outside the entrance are two benchmarks. Both are the locations of the intersection of the same latitude and longitude, but they are about thirty meters (one hundred feet) apart. What gives?

I took the free GPS course from the USGS last fall and was introduced to this dilemma. How can the same coordinate be in two different places? The answer? Datum shift. One coordinate is in the old NAD 1924 datum, the other in the newer NAD 1984 datum.

What’s a datum? The datum is the reference model of the earth used to define a location. The first datums assumed the earth was a perfect sphere; later ones used a perfect ellipsoid; even later ones roughened the surface of the ellipse, and so on. In truth, there are hundreds of datums, some abandoned, many not. Almost every country on the planet as its own datum. We use the North American Datum, or NAD, and it establishes the base reference model of North America that is used for spatial measurements here. The newer datum corrected some gravitational and modeling errors in the earlier datum. (Quiz: does the Middle East use NAD 1984 or NAD 1924? Answer: Neither: They use WGS-84.)

So what’s this mean to you, the Colorado mountain hiker/biker/skier? Plenty if you use GPS and USGS topo maps (or maps such as Trails Illustrated that use the USGS maps for their bases.) The USGS topo maps were printed before the datum shift; they are NAD 1924. However, your GPS defaults to the NAD 1984 datum. When you are locating yourself on the map using the GPS coordinates, it might appear that you are in the stream, even though you know you are on the ridge above the stream. You grumble and gripe, and decide to throw out the GPS, or the map, depending on your mood that day. But both tools are right, they are just giving you information in different systems.

So what to do? Make sure your map and GPS are in the same system. If you are clever enough to use online mapping, National Geographic TOPO! for instance, the maps have been adjusted to the NAD 1984 datum. If you are using your old stash of paper maps, or buy new ones, check the datum, usually found in the lower left corner text. If the datum is NAD 1924, adjust your GPS so it shows you coordinates in the NAD 1924 datum. This is necessary whether you use lat/lon locations, or better yet, the UTM meter coordinates. And if you don’t know how to use the UTM grid, contact the USGS and take the free class. Call (303) 202-4640 for information about the next available class. P

Susan Rhea is an information technology specialist for the United States Geological survey. She has been a CMC member for twenty years.