Monday, December 14, 2015

Your GPS Watch May Be Overestimating Your Mileage

Picture by link_alicante from Flickr
Recently one of my patients mentioned that a particular race he finished seemed to be longer based on the reading on his GPS (global positioning system) watch. He said that he did not run the corners in the race at the widest bend nor did he weave in and out while overtaking other runners. Neither did he take a wrong turn during the race. He was upset that a certified course was "inaccurate" according to his GPS watch.

Well, recently I happened to chance upon an article on how distances are measured via GPS watch/ phones etc and I told him what I read.

A GPS device can measure distance as orbiting satellites communicate with your watch/ smartphone that you run or walk with.  The communication is not continuous but a frequent "check in". While running or walking, your GPS device collects a GPS track. A GPS track consists of a sequence of consecutive GPS positions.

The GPS device does not monitor your movement continuously, it monitors your location at specific times and then measures distances between each location. Potentially, this leads to two types of errors.

The first error is the interpolation error. If the GPS watch/ phone you run with does not show your position frequently enough, it will underestimate the distance you ran.

The second error is a measurement error. Each time the satellite tries to pick your location, it can be slightly off. When your watch/ phone tries to measure your position between your last two positions, the distance will be inaccurate.These tiny errors add up the longer you run.

Hence, if your GPS watch collects too many locations as you run, your distance tracked will be longer. If not enough positions are collected your run distance measured will be shorter,

For this study, the researchers had their subject walk on a pre measured track using a a GPS device that tracked positions every second. A very low quality GPS device was used deliberately as they wanted the error to be visible. The researchers noted that with a better quality device the error will be less but will still be present.

As technology improves, these errors will be minimised though.

I've never owned a GPS watch and have never ran with one so far. Looks like I probably don't have too.


Ranacher P, Brunauer R et al (2015). Why GPS Makes Distances Bigger Than They Are. Int J Georg Info Sci. DOI: 10.1080/13658816.2015.1086924.

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