how to check if your GPS running app / watch is accurate

Recently there was an article in NYT which claimed GPS watches can be unreliable and give you the wrong distance. One example is running around a track and then downloading the map data to your computer. You’ll see that none of your laps will actually overlap and they may not even look like ovals. This is mostly true but there are always workarounds. I’m not talking about a certain kind of GPS watch but more about GPS apps for tracking. For example with Google’s own My Tracks app there’s are several settings that help you keep tabs on your accuracy. The most important one of these is the “time interval” which is defined as the smallest distance between two recordings of your position. You want this value to be as small as possible (and it is by default set at 1 seconds we believe).

Another important setting is the distance interval. If your path is filled with switchbacks or just a lot of turns this setting may help you smooth out the turns and make it more realistic. The recommended setting for this is by default 5m but if you’re crazy about collecting the most realistic data you should set this to it’s smallest value which is 1m.

A third setting that helps you keep track of your accuracy is the “GPS accuracy”. Google My Tracks has this setting at 200 meters by default. This means when that blue circle around you is at 200 meters or less your data will be recorded. Now to be honest 200 meters is a long distance and may make a big difference if you’re running. You can set this at 10m accuracy which is a pretty darn good signal. But this means your phone will stop recording your workouts when the signal gets even a little worse. I’d rather have the app record my workout rather than it stop and complain about the signal.

One final thing you can do to test your GPS devices’ accuracy is to measure your path on Google Earth. In one window bring up the path you uploaded from your tracking app and on a another window bring up Google Earth. Now go to Tools->Ruler->Path from the menu bar. Using your mouse start clicking on the path you followed. Make it as accurate as possible. Now compare that distance to the distance reported by your GPS app. Is it off by a lot? Probably not. 99% accurate? If that makes you happy great. Nothing to worry about. If not you can bring up Google Earth every single time, correct your path, and record the actual distance.

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