Why Your GPS Watch Doesn't Match the Race Distance

GPS Inaccuracies

You may be saying to yourself, "OK, so the course is accurate, but so is my GPS. Why don't they match?"

While the GPS has come a long way, it's still not 100 percent accurate. GPS devices work by receiving signals from satellites—triangulated signals to be specific—meaning that in order to attain a reading, the GPS unit must have a clear view of at least three satellites.

Tall buildings, dense foliage and other structures can affect the signal and cause errors in accuracy. Satellites that are in poor geometrical relation to each other can also be a source of GPS signal errors.

Another thing to keep in mind is that GPS devices do not check their position constantly. While some check every second, others check every 20 seconds (or more). Satellite reception can be lost in those time periods and, if a runner is moving quickly, a portion of their run could be measured incorrectly.

"It's not uncommon for casual runners to be as much as two-tenths of a mile long on their watch."

Your Personal Route

But what if there aren't any tall buildings or dense foliage? The next factor to consider is how you ran the course itself.

When a course is measured for distance by USATF, it is measured through what they have deemed the "SPR," or the shortest possible route that a runner can possibly cover. In many races, it's nearly impossible to run the exact route that the measurer used due to other runners on the course, making wider turns or stopping for a Porta Potty or water station. This is one reason many runners aim to run the tangents. Running a course with plenty of turns could potentially lead to a big difference between your GPS and the course distance.

In fact, matching the course distance is anything other than an easy task.

"Only seasoned runners that are able to run the tangents should expect their watch to match the distance," says Nick Zivolich, the race organizer for the Best Damn Race series in Florida. "It's not uncommon for casual runners to be as much as two-tenths of a mile long on their watch."

Want to know if your next race is USATF-certified? Visit usatf.org.

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About the Author

Beth Shaw

Beth Shaw is a USA Track and Field Certified Coach and a US Masters Swimming Certified Coach who has completed numerous marathons, road races and an Ironman triathlon. Beth writes a fitness blog called Discombobulated Running and has contributed to Women's Running. She also coaches runners to reach their athletic goals. You can follow her on Twitter.
Beth Shaw is a USA Track and Field Certified Coach and a US Masters Swimming Certified Coach who has completed numerous marathons, road races and an Ironman triathlon. Beth writes a fitness blog called Discombobulated Running and has contributed to Women's Running. She also coaches runners to reach their athletic goals. You can follow her on Twitter.

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