Thursday, December 22, 2011

News flash - GPS watches aren't accurate

Graphic from 12/19 NY Times article
Photo credit: Martin Strauss, via Garmin and Microsoft
 Today's run (street): 2.5 miles
There was an article in Tuesday's New York Times about the accuracy (or should I say, inaccuracy?) of GPS watches. The writer made the same point that I've been making for years, that GPS technology does not provide exact measurement. The same can be said for smartphone GPS apps that show lots of metrics and graphics, but also under-count distance. The thing is, if your distance is off, so is everything else, including pace.

I'm a bit of a belt and suspenders guy when it comes to tracking my runs. I use a Garmin FR210 GPS watch to capture my run metrics and route. But later, I usually measure where I had run using Gmaps to get the most accurate distance. This way I can calculate my true pace. Why use the Garmin if I know it's inaccurate? Good question. But just try taking my Garmin away from me...

This morning I got back outside for my early run and was rewarded with dry roads and mild temperatures. There was wind coming from the north that, when traveling head-on, made things chilly. Most of my route went either south or east so it wasn't a problem. I managed an average pace, though my PE (perceived effort) was higher than that result. Then again, when you get your performance information from a GPS watch, you never know what to believe!


  1. You might find DCRainmaker's post, commenting on the NYT article interesting.

  2. I was going to mention that post, actually. I left a comment on the site yesterday saying that I disagreed with his position that GPS would tend to overstate, rather than understate mileage. That's certainly not been my experience...

  3. I have a Garmin Forerunner 305 and my experience has been that it overstates the distance a bit.

    On a 5k It will usually record 3.14 instead of 3.1. On a marathon the watch records around 26.44.

    Part of the overage can be attributed to the fact that the person that lays out the course assumes that you run tangents around curves. Most runners of course don't run the optimum path. My wife has the same watch and on occasion we differ by .01 miles on a 5k.

    I have mapped my data into Google Earth and sometimes the watch is a bit off. It will appear that I ran on somebody's lawn instead of the street. On the track it sometimes indicates that I ran on an outside lane when in fact I stayed on the inside lane for the entire run.

    The technology is not perfect but the % error is still very low.I find the distance, pace and heart rate info quite useful.

  4. Frankly, looking at the concept of a GPS unit calculating distance and being as accurate as they are you have to admire the work behind the devices. You're running, with a unit on your wrist which is moving back and forth, going under trees, darting around obstacles, etc., yet the device can still maintain a signal lock and give you a distance that is off by a few hundredths of a mile. Impressive is the word.

    Frankly people that complain about GPS watches not being accurate against a measured course (or against something like Gmaps which uses the route a vehicle would traverse a road to calculate distance)are full of it. You know the race course is 5k, 10k, 13.1 or 26.2 miles...all that matters is the time your watch is displaying. For training runs, as long as the watch is consistent in its percent error over different then you experience no real comparison issues.

    As for the original article...I can't remember reading it, but did the author allow her watch to properly locate the satellites before starting her workout? I'm assuming she was in the city...takes some time to acquire signal.

  5. My FR210 generally records distance about 3% less than actual. A look at the route on Garmin Connect explains that, curves are often truncated to sharp angles that add up to less distance.

    My usual morning route is 2.53 miles which I have exactingly measured with both GMAPS and Google Earth. But my Garmin will commonly range between 2.39 and 2.5 miles.

    I've used MotionX, AllSport GPS and half a dozen other smartphone apps with similar results. The variance is a given, but I accept it.

    The article didn't mention whether the author locked in optimally for satellite reception but I think she mentioned some units do a better job of that than others. I just wait until my watch goes from the acquisition screen to 00:00. I assume I'm locked in at that point.


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