So how far did you run? – GPS Accuracy

GPS_Satellite_NASA_art-iifAfter the Endurancelife Coastal Trail series event on Saturday quite a few runners were exclaiming that the route was short at around 25.8 miles. What’s 0.4 of a mile between friends? Well most Endurancelife marathons are between 27 and 29 miles, so to have one short was a bit of a surprise. I got home and measured the distance using some popular mapping software and it came in even shorter at 25.3. It got me into thinking about how accurate the way we measure routes actually is.

Most of us take the number on our GPS wristwatch as being 100% true, but actually this is far from correct. Rather than do a lot of legwork myself I did a quick bit of research and found this article on measuring routes on The Good Running Guide. I would strongly recommend giving it a quick read as it is written in plain English. The most interesting revelation is that they found GPS watches to give an end result that was between 4% to 8.4% out and that was with a good view of the sky. So it looks like GPS accuracy isn’t quite what you might think.

In another article on the New York Times Website similar inaccuracies were found. When explaining the results of someone running along a twisty wooded trail they said the following:

The actual distance was 6.6 miles, and his actual pace was 7 minutes, 37 seconds a mile. The watch that did best said he ran 6.45 miles at a 7:47 pace. The one that did worst said he ran just 5.5 miles at a 9:08 pace.

Now, the further you run the more inaccurate the result will be. Don’t give up though, you can use them with features such as the elevation correction on the premium subscription at www.trainingpeaks.com to help make them more accurate if you need to. For most of us, the fact that they are accurate to within ten feet is more than accurate enough. These little errors really do add up quite rapidly though, so don’t go running to a race organiser to tell them that they measured their race wrong just because your GPS device tells you so. To be honest, even if your GPS matches those of your competitors, that still doesn’t mean they are 100% accurate, after all the devices rely on the same technology and you ran through the same areas with the satellites in similar positions, so they probably recorded similar errors to yours.

An example: On a hilly and occasional wooded trail marathon you can expect your overall GPS recorded distance to be between 1.04 and 2.08 miles out if we accept that it will have an inaccuracy of between 4% and 8%. Most of the pages that I read also showed the readings to usually be short, rather than long for specific running devices, so take heart that you have probably run faster and further than your watch says.

My recommendation: Don’t throw away that GPS just yet, it is still a great training tool and gives you plenty of useful information, but maybe don’t rely on it quite so much during actual races other than for heart rate information. Instead turn your attention inwards and rely on your body to tell you what the correct pace is, learn to run by feel and don’t worry about how many miles are left to run. By the end of the race it doesn’t matter how quick you were during any particular mile, what matters is your average pace over the entire race.

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