How accurate is your GPS?

On most of my (our) bikes, I use both an analogue cycle computer (that relies on wheel revolutions to record the ride) and a GPS (usually a Garmin).

However, I have recently acquired another bike (but more of that in a future post) on which I use only the Garmin and a tracking app on my phone. I decided that fixing an analogue system was too ‘yesterday’. So today, and not for the first time, I used only my Garmin and Strava on my phone to track my route, and interestingly,  they came up with very different statistics. Different enough, in fact, to hold in question the accuracy of GPS technology. But there could be a simple explanation…….bear with me.

Here is the Garmin map:

Ramsey 70km.1It records me covering 70.90km, at an average speed of 24.3kph, with an elevation gain of 310 metres, and a maximum speed of 51.5kph.  For a flattish ride out to the fens, the elevation gain is quite startling………..but it is barometric, and not necessarily accurate.

This is the Strava map:Ramsey 70km.2

Strava, on the other hand, records me covering 69.90km, at an average speed of 24.9kph, elevation gain of 381 metres, and a maximum speed of 45.5kph.

So Strava had me cycling 1000 metres less than the Garmin, had me climbing 71 metres more, at an average speed of 0.6kph faster, but with a maximum speed of 6kph less.

I can’t explain most of these anomalies, but I can add that the auto-pause functions of both devices would have made some difference. The Garmin had me cycling for 2:55:20, and the Strava for only 2:47:38. That critical difference of some 8 minutes would have affected the average speed, and might have affected the divergence in distance, especially if one device took a few more seconds to kick back into activity when moving off from stationary.

Funnily, when I did use an analogue system, I always relied more on the wheel revolutions for giving me a more accurate record of my ride. Maybe I’ve not been so wide of the mark after all……….





About Frank Burns

Looking for the extraordinary in the commonplace………taking the road less travelled……..striving for the ‘faculty of making happy chance discoveries’ in unremarkable circumstances. Click on the Personal Link below to visit my webpages.

Posted on August 23, 2017, in Uncategorized. Bookmark the permalink. 4 Comments.

  1. All I use anymore is analog. Grew bored with the gadgets. 👍

  2. 2 things – first it depends how often they sample your position – the less often the more smoothed out the “curve”

    second your Strava plot was obviously not functioning properly at the start – unless you cycled straight across the field up the “hill” but somehow managed to kid the Garmin plot that you’d stuck to the road

    As to analogue – tiny miscalculations of the wheel circumference wold probably make a bigger difference – it might even change as the temperature changed

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