Why is my GPS Watch Off?

Photo by Flickr user: 1yen

Periodically, I receive emails asking about the accuracy of GPS running watches. Along the same lines, runners are wondering if their watch is accurate because they just completed a 5K race, but the GPS watch reads something other than 5,000 meters.

So, I was going to write a post that addresses these questions. However, I recall a couple of great articles on this topic. First, the NY Times made a half-hearted effort to explain the discrepancy of GPS watches:

GPS Watch Can be an Unreliable Running Partner

An excerpt:

It seems clear enough that a GPS watch is not very accurate, yet online runners’ forums, like one at the Web site of Runners World, are filled with comments from confused athletes who rely on the devices.

However, the article was not written as a true scientific analysis to explain why GPS watches calculate distances differently. Thankfully, someone else was able to provide a logical response:

Why the New York Times GPS Running Article Missed the Boat by DC Rainmaker

Part of the response:

The challenge here is that I’d ask first – what were the watches in question?  What brands, devices and software versions?  For example, was the one that was off by .42 miles (97.3% accurate) an older model from 5-6 years ago, or was it more recent?  Who made it?  What firmware version?

Regardless of all the analysis, the benefits of a GPS watch are many. Having an understanding of how far  you’ve run and how long it took will help you improve your running.

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