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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 ( 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 ( 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.

E-mail your comments to me at [email protected].

Joe Desposito

Company: EEPN

Product URL: Click here for more information

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