When a Good GPS Goes Wrong

James Rebholtz, USF&W, takes a GPS reading. Photo by Bruce Henrickson at http://search.ahp.us.army.mil/search/images/index.php?search=gps

James Rebholtz, USF&W, takes a GPS reading. Photo by Bruce Henrickson at http://search.ahp.us.army.mil/search/images/index.php?search=gps

Considering the importance of knowing where you are, if you depend upon a GPS for that information you should be confident what it tells you is right. Sometimes all you get is an “acquiring signal” screen. That comes with the territory–if you’re in dense canopy or deep canyons you expect to have some trouble.

But what about when you emerge from the woods? How much of what the device tells you about your trip is accurate? Tons of information appears to be available–trip time, trip distance, average speed–and it may be important if you have to figure out how to get back.

I really enjoy using mine on runs out here in the country–I can keep an eye on my speed, mark the miles as they tick away, and it’s a handy record of progress (or lapse). In the winter it’s been very accurate, but this Spring when the leaves came out fully and several stretches of road we run became tunnels of trees, I noticed something odd. I expected to disappear from the radar, so to speak, and got the “searching for signal” warning from time to time. What I didn’t expect was to be told that my average speed for the trip was fifteen miles per hour. Checking the other information I discovered that about half a mile of distance was completely missing from the track.

I don’t quite have an explanation for the fifteen mph error and assume it’s a cascade issue resulting from something else. The GPS made a mistake in triangulation and having lost a critical piece of info, went way wrong. Here’s what happened:

The satellites were low in the southern sky. As I went into one section of woods the satellites lost track of me. When the GPS picked them up again, it used them to calculate my new position and drew a straight line between the two points. But, I hadn’t traveled a straight line. I’d gone around a corner and traveled considerably farther than that straight line distance.

Here it made very little difference to me, but if I were out in the woods trying to figure out how to get from point A to point B, this might slip past me and screw things up. I’d expect to be moving faster and over less distance than I actually was.

Fortunately, the clock didn’t lie–trip time was unaffected by the mistake. And for the record, that’s the fastest fantasy mile I’ve ever run.

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