The Reason for Different GPS Positiions for the Same Location

You Gotta Be Kiddin’

Can your chart (or Google Earth) give you one position and your GPS another?

Some while ago I received a letter from William P. Carter, an observant Virginian with a serious interest in all things nautical. He had a question that conveniently points out a foible of modern, GPS-reliant navigation, one that seems just a tad ridiculous, at least at first blush.

Here’s the question: “I recently located a point of land in the James River in Virginia with GPS lat/lon coordinates at the request of a friend of mine. Upon returning home, I looked at the same point on Google Earth and noticed that the cursor displayed coordinates that were way different from the ones I’d just recorded. The latitude was close—the difference was only 50 feet. But the longitude was well over a mile off. I went back with another Garmin handheld GPS and got almost the same results as I’d got the first time around. Is Google wrong or Garmin?”

Turn to the experts for answers, I always say. And as luck would have it, I’ve got a friend who’s worked for the United States Geological Survey (USGS) for decades now and, besides being an authority on various forms of cartography, he’s pretty good at explaining quirky, oddball stuff as well.

Heck yes, he said when I mentioned Carter’s question. Bumping into sets of dissimilar lat/lon coordinate pairs generated by dissimilar types of cartography is fairly common amongst navigators these days. And the reason’s simple: Cartographers have been creating cartography for ages, and in that time they’ve used a host of different geodetic models or datums to do it.

Until about 20 years ago, he went on to explain, the most commonly used datum in the United States was the North American Datum of 1927 (NAD27). But the National Geodetic Survey defined a new and improved version in the early ’80s known as the North American Datum of 1983 (NAD83), and more variants followed soon after.

Confusion results, my friend concluded, because so many datums are in use around the world today. Indeed, many modern GPS receivers default to NAD83 but have a menu setting that allows users to switch to any number of other datums, among them the WGS84 Datum that Google Earth uses. After all, relying on various types of cartography with incompatible datums can get a guy into a whole pile of trouble in a hurry!

So yeah, the physical location of a point on the planet—whether along the shoreline of the James River or anywhere else—may remain constant (barring unforeseen circumstances) but the cartographic representation of that point is frankly up for grabs these days, based on which datum has been either defaulted to or chosen.

So Mr. Carter, the answer is that Garmin’s computed lat/lon for your controversial little spot is probably correct. But then again, Google Earth’s is probably correct as well. Pretty crazy, eh?

(913) 397-8200.

This article originally appeared in the June 2011 issue of Power & Motoryacht magazine.