Why your GPS Watch Measured the Race Course Wrong (And What to Do About It)

Have you ever done this before? You’re running along, getting close to the finish of your race. It’s the end of the race, so every part of your body is starting to hurt—even random parts like your little toe and your ears. But you’re pushing through because your watch says you only have ¼ mile to go. Eventually your watch beeps over saying you’ve hit the total race mileage, only the finish line is nowhere to be seen! What the what?! After running for several more minutes, you finally find cross the finish line with your watch saying you ran an extra ½ mile!

What just happened? Was the race course long? Did you get lost? Sign up for the wrong distance race? Nope, nope, and nope.

Why your GPS Measured the Course Wrong

Reasons Why Your GPS Watch Measured the Course Wrong

You didn’t run the tangents. Race courses are measured by taking the most direct route, so you add extra mileage to your race if you don’t follow their exact path. Whenever the course is turning, you want to run the most direct route possible. If you’re on the right side of the road and have a lefthand turn coming up, you want to slowly make your way towards that turn. Treat tangents like you’d treat a bear: no sudden movements. For a great visual of running tangents, check out this image from Runladylike. If you follow that link, she has some great tips for running the tangent.

Image from Runladylike
Image from Runladylike

Water stops. Let’s say you’re running down the middle of the road, rocking out to some Bruno Mars in your head, when you notice that you’re about to an aid station. So, you veer off to the right to grab a cup of water, then head back to the middle of the road to attempt to drink it, where we all know you actually drink less than gets spilled down your shirt (why is running and drinking at the same time so dang dong tough?!). Your necessary journey to the aid station added a few extra feet to your race. Now you’re running a bit more than the race distance, and your shirt and hand are sticky from the Gatorade you just got everywhere. #winning

Buildings. Bridges. Steep hills. Other people’s GPS watches. All these things can interfere with your GPS signal and cause your watch to get a bit wonky. The Chicago Marathon is notorious for peoples’ watches going haywire in the first mile after passing through an underpass, with some people having their watch clock their first mile up to a half mile short! GPS watches are not flawless in any circumstance, and a big city race can be a recipe for GPS disaster.

General GPS error. Even in ideal circumstances, GPS watches will pretty much always have some amount of error. Don’t believe me? Go run a mile on a track (make sure you do an actual mile, not 1600 meters), and see how far your watch says you ran. You might be shocked at how far off it is! If you’re interested in the accuracy of different GPS devices, Fellrnr did an in depth scientific comparison of many of the big name GPS devices and wrote his results into a fantastic article.

How to Deal with GPS Error in a Race

Turn off the auto-lap feature. I hate having my watch click over to 20.00 miles when I’m still several minutes from the course mile marker. It’s defeating and frustrating. So, I set my watch to only auto-lap every 10 miles and manually hit the lap button at each mile marker. I keep my watch set to only show information about my current lap. This way I still have access to my current pace (though I try to only look at my pace at each mile…obsessing about pace throughout a race is no bueno) without feeling like I’ve run further than I have.

Assume you’ll be running a bit longer than the course. As you prepare for a race, recognize that you will probably be running a bit further than the race distance and factor that into your goal pace. For instance, say you’re hoping to break 4 hours in the marathon. You need to average 9:09 pace each mile, but try to train for a bit faster than that to make up for the extra mileage you run because of missed tangents and water stops.

Choose a straight course. My watch was almost spot on when I ran Grandma’s Marathon, a point to point marathon that goes mostly along one road with just a handful of turns at the very end. The fewer turns in a race, the less you have to worry about tangents adding mileage to your run.

Choose a small to midsize race. You know what else adds extra mileage to a race? Bobbing around all the slower people who started in front of you, which can be a major factor in bigger races. Don’t even get me started on the British 10k, a huge 10k with no corrals and the charity runners/walkers start first. I’ve never had to dart around so many people in my life! Big races are fun, but I personally prefer to use midsize races when I’m gunning for a PR.

Don’t claim a PR from your watch. At the end of the race, your time and mile splits are according to the race’s timing devices – not your watch. As we’ve discussed here, GPS watches can be wildly inaccurate. While it’s a bummer to have your watch claim you actually ran 26.8 and that you set a PR at 26.2 miles, it doesn’t count. So don’t try to claim it.

Don’t complain the course was long. Don’t go around telling everyone that the course was long. I’m sure you can find several other people whose GPS watches measured the course wrong too (I just gave you all the reasons why!), but that doesn’t mean diddly squat. The course directors have specific, accurate ways of measuring courses. I can assure you: they are right, your GPS watch is wrong.

A straight course helped me to set a shiny, new PR at Grandma's Marathon
A straight course helped me to set a shiny, new PR at Grandma’s Marathon

Your turn…

Has GPS ever given you heartbreak at the end of a race?

How do you use GPS devices? Training? Races? Never?

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