Going The Distance With GPS

One of the runners in my running club is using a GPS watch and has gotten into the habit of telling me about the discrepancy between the distance reading on his watch and the course distance. Usually, I tell him that the GPS system isn’t that accurate. From the look on his face, I can tell he doesn’t believe me.

Then, at a recent holiday get together, I found myself in a conversation with Juan, the aforementioned runner, and another guy, Rob, who also has a GPS watch. Rob, a lawyer with a penchant for exact calculations, once explained to me that a formula I was using for race pace was off by 0.22% since I was using 3.1 miles for a 5K distance instead of 3.10686. Now, Rob was trying to make the case that as a runner moves his arm back and forth during a race, it affects the GPS readings and causes a cumulative error in the final reading. I couldn’t buy into this explanation, even though it fortified my case against Juan.

The real problem stems from the fact that the courses my running club (www.lirrc.org) uses are not actually certified. We use many different distances, too, from 5K to a half marathon, and therefore we mix and match different loops to come up with the final distance. The most recent was a 5-mile race with one 1-mile loop and two 2-mile loops. Juan’s complaint was about the 1-mile loop. Typically, we use a Jones counter to determine the distance of a particular loop. This is a mechanical device that attaches to the spokes of a bicycle wheel. Knowing the circumference of the wheel and the number of revolutions gives you the distance travelled

As for GPS, Timex makes a watch called the Ironman Triathlon Speed + Distance System (www.timex.com). The system consists of a GPS receiver that you wear on your arm plus a watch for your wrist. According to Timex, the system has 99% distance accuracy in environments with a clear view of the sky. This 1% error means at least 50 meters in a 5K race. But Timex often uses the word “exact” in its promotions, so I’m not sure if I’ll ever be able to convince Juan that the Jones counter is a more reliable measure of distance than the GPS system.

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—Joe Desposito