Is Your GPS Watch Hurting Your Running?

We rely on our GPS watches to track running distance, overall pace, instantaneous pace and other variables. Is your GPS watch really accurate?

Training for Grandma’s Marathon was not going well for coaching client Diann.  She was experiencing difficulty completing workouts that should have been doable.  She was chronically tired.  Her lower back and knees were painful.   She was forced to take frequent breaks from training to prevent her nagging injuries from developing into full blown injuries that would force a long layoff.

This didn’t make sense.  Based on her fitness and experience, her workouts should have been doable; the training load should have been manageable.

We searched for a solution.  On a 7 mile (11K) run, Diann insightfully observed:

“Here’s the aha – at the end of the run my (GPS) watch said we ran a 10:00 (6:15 min/km) average. Throughout the run I saw 10:30 (6:34/km) frequently floating by for pace, so I assumed I was running pretty casual.  But my girlfriends’ GPS watches said 9:15 (5:47/km) to 9:22 (5:51/km) pace.  That’s a big difference.   I believe my watch has also been contributing to the injury issue and I’ve been at times inadvertently running too fast”

I suggested Diann start training “old school”.  Shut off pace function.  Run by perceived effort.  Calculate pace using pre-measured distance and time.

She quickly turned her training around. She consistently completed her workouts on target.  Her nagging injuries cleared up. Her chronic fatigue lifted.  She ran a 4:01:40 PB at Grandma’s.

What’s going on?  We rely on our GPS watches to track distance, overall pace, instantaneous pace and other variables.  For many of us, our GPS watches are an indispensable training aid. Aren’t GPS watches really accurate?

Unfortunately, the answer is “no”.  GPS technology is far from perfect.

In Gina Kolata’s article GPS Watch Can Be an Unreliable Running Partner that appeared in the New York Times, the author writes that race directors know in advance they will receive post-race complaints about the “inaccuracies” of the race course.  In every case, the course is accurate, the GPS is not.

The problem is the technology is not perfect.  “The technology is improving.  But some inaccuracy remains”.  Trees, clouds, tall buildings all interfere or block the satellite signals that GPS watches use to calculate pace and distance.

Courses with lots of curves are particularly problematic.  ”If you lose the signal as you go around a curve, your device will draw a straight line from where it last saw you to where it found you again. The distance around the curve will not be tracked”.

Martin Strauss, the author’s friend, is a professor of mathematics and electrical engineering and computer science at the University of Michigan. He demonstrated the imperfection of GPS technology by running around the university track five times.

“When he downloaded the GPS data onto his computer, every loop around the track was a little different, and none were oval.  In fact, not one of his paths was even curved — they were short segments of lines connected to resemble an oval. Yet he had run in the same lane”.

What to do if you suspect your GPS is leading you astray? Follow Diann’s lead.  Run with several friends, each recording distance and pace.  Compare your data.

Or, disable/ignore your pace function entirely and enable your “perceived effort” function.  Check pace occasionally using pre-measured distance and elapsed time.  After a few runs, you’ll find your perceived effort is just as reliable. And you’ll enhance your mind-body connection.

Throw your GPS watch out? Not at all. On most runs, it will be accurate enough.   But don’t rely on it entirely.  It should be one of the inputs you consider when training. Don’t let it overrule your inner GPS.

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2 Responses to Is Your GPS Watch Hurting Your Running?
  1. Sarah
    June 3, 2015 | 9:07 am

    I feel like a more accurate title would be “Is relying on pace hurting your running?” It’s not the GPS watch per se that’s hurting our running, but our giving in to the temptation of either paying attention to instant pace during the run (which can be all over the place) or caring about what that pace is. I set up my watch’s screens to show HR (although I can usually feel where I am) and only refer to pace in very specific situations. I love to have the data afterward to pore over, and even though it may or may not be consistent with a running partner’s GPS, it’s usually consistent with other runs I’ve done with the same device.

  2. Bennett
    June 4, 2015 | 7:56 am

    Thanks for your comment. I like how you use your GPS watch to evaluate your workout once it’s complete, rather than relying on it for instantaneous (and sometimes inaccurate) feedback during your workout …..except for specific situations. You’ve struck a healthy balance.

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