A little test of GPS accuracy which I thought I would share.

At the end of some runs and races, I often wonder why the official distance on the race organisers website and my Garmin have a little disagreement.  This is particularly true on trail events, where the organisers themselves are likely to rely on their own GPS’s and taking averages from participants GPX files.  For example, at one of the CTS endurancelife races, my garmin came out with 26.4 miles and their website showed 27.8.  Ok, so for some frequent marathon runners and certainly ultra runners you won’t care, BUT when you know you have £400 strapped to your wrist (or in my case my pocket as I have a Garmin EDGE 800 designed for a bike) and knowing there are dedicated satellites staring down on you – you’d be forgiven to think they should be bloody accurate.

So how much of the Garmin should be believed and how much should you take for granted.  Well I did a little test.  Not conclusive and only based on this one test run, however I Hope you find it interesting.


The Route:  Along the sea front in Poole/ Bournemouth taking in three hills, completing each hill 10 times.

Stats: GARMIN –               13.99 miles, 3,560ft climb

Stats: GOOGLE MAPS –  16.10 miles, didn’t check the climb but if the distance was different you can bet the altitude would have come up different.  Furthermore, look at the picture below of the Garmin – it’s clearly confused as I started and finished at the same sot, but it thinks I finished lower (which given where I live would mean i live under water)!

Results: I have shown below the whole route as per the Garmin and below that the Google maps equivalent (note on my google maps I have not included the 10x hill repeats as my footsteps were virtually identical on each repetition).  Then shown zoomed in sections of each part to see how accurate it was against the reference in Google.

Whole route summary  – note the thicker blue lines are where I have completed that section 10 times.

Zoomed section of West Undercliff – important to note that I ran almost exactly the same line each time although this does show some incorrect variance to the route.  The single track to the left is correct as I rejoined the beach path via a different path.

Zoomed section of Branksome – in this 10x section there was a very tight switchback to the car park which didn’t get picked up all that well – see the second picture down for the actual line taken

Zoomed section of Canford Cliffs – this is aweful.  The line taken each time was exactly the same (there are two routes below, a left and right path which I alternated on) and so not sure where all this variance came from

would love to know your thoughts!?

4 Replies to “GPS accuracy test”

  1. To be honest, I’m pretty impressed with the accuracy the Garmin is displaying here. If you go onto GarminConnect and upload the track, you can “correct” it, which usually does a pretty good job of sorting out the altitude (although it seemed to delete a hill at Exmoor – the first such issue I’ve had with it). I work with GPS-related technologies, so I’m perhaps a bit more forgiving than the average user. The decoding chip in the Garmin is typically accurate to within <5m if you sit still in an open field for an hour (try it, and you should get a trace that looks like you've been drunkenly swaying). Normal things in the world that cause the accuracy to drop to ~10m are high buildings, thick woods, and large bodies of water. These cause either a loss of satellites (less accuracy when you only get readings from a few as opposed to many), or a distortion of the time for signals to get from the satellite to the watch, which creates a shift in distance calculated by the watch. Then, it's all about how good the algorithm is that Garmin are using to correct / ignore the distortions. If you have one of the new iPhones, then you quite possibly have both a more accurate chip and an improved algorithm from the app supplier.

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