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Therese Iknoian Wednesday, 28 December 2011 07:57 TWEET COMMENTS 3

GPS Accuracy?

Ask the Coach

I have run a few 50-milers, where the race website claims, for example, 9500 feet of climbing, but my GPS ...

Illustration by Jeremy Duncan

I have run a few 50-milers, where the race website claims, for example, 9500 feet of climbing, but my GPS will record 13,000 feet. What accounts for the difference?

—Deb Paquin, Folsom, CA

Before you cry foul, find out how the race determined cumulative climbing and how accurate that method is. Without that knowledge, though, Coach will cover GPS accuracy and use.

"GPS units are not, repeat not, altimeters," says Michael Hodgson, author of Compass and Map Navigator (Globe Pequot, 2000), a longtime trail runner and outdoor expert. "Never rely on a GPS for elevation. That's what altimeters are for." While horizontal-distance readings will usually place you within 10 to 25 feet of your actual location, possible vertical-elevation errors can be up to 300 feet. Multiply that error with several hundred readings over a 50K and, well, you get the picture.

For pure distance readings, a modern GPS is more accurate, especially since these days they take readings from so many satellites and can compensate for intermittent signal losses. If you set the GPS to take more frequent readings (e.g. every 10 seconds vs. one minute), it will be even more accurate, but the disadvantage is a higher battery draw.

What messes with the accuracy? "Canyons of rock, dirt or even skyscrapers or a thick forest can distort or inhibit a signal," says Hodgson. If you lose the signal when you are on a trail with lots of turns, the GPS will automatically assume a straight line from the time it lost reception until the time it picks it back up.

The same issue may occur with descents and ascents since it may project a straight line across the canyon rim, when, in fact, you doubled that distance with the downhill and then the grunt back up. Remember, GPS accuracy varies widely, depending on design, environment, settings and use. Read your manual to get the most from your GPS.

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