Is my GPS measuring distance correctly?

Q. Hi,

I always reset my GPS tripometer at the start of every walk.

At the end of the walk it always states that I've walked a lot further than I actually have.

Is this a common thing with a GPS or have I got a faulty one?

Thanks

Matthew Jones, Swansea

A. GPS is nominally accurate to about 20 metres (though often down to 3 or 4 metres) if it's got 4 good satellite signals. If the receiver loses some or all of those signals (reception is at it's best when you can see a clearly to the horizon in all directions, the signal can be degraded in canyons, under trees or even a little by heavy cloud) odd things can happen, just what depends on the sophistication of the device hardware & software. My old garmin could sometimes place me hundreds of metres off course, my new one does a lot better. Friends with wristwatch GPS or in mobile phone experience far more issues than on a full size dedicated up to date hand-held device. One of those found that sitting below a cliff (implying restricted satellite-view) for his coffee he'd apparently climbed 200 metres. Smarter software might calculate that if the average speed of movement is 5km/h and it changes briefly to 50km/h, maybe the data is faulty - if the speed change is consistent then OK, you were walking but now you caught the bus, if it's a brief blip and you seem to have returned to a slightly earlier location then, yes, it was a bad data point which should be discarded.

Whilst 20 metres on the horizontal plane doesn't make a great deal of difference (assuming you keep your eyes open and don't step off the edge of a cliff because the GPS told you to...) 20 metres climb is distinctly noticeable. We are finding that on a 10km walk with 300metres ascent the poorer GPS devices may accumulate a difference of maybe 100-200 hundred metres distance but also, and much more significant, 100 metres climb.

My usual practise is to plan a route on digital mapping - because I don't carefully follow every little twist and turn the distance/climb is an under estimate (especially in the vertical axis). At the end of the walk I get what is probably the most accurate reading, but it includes when I went off track for a pee or misssed a turn and had to backtrack (I don't navigate by GPS, I just use it for an occasional check). When I get back I import the track from GPS to digital map, apply smoothing and corrections and end up with what I regard as the best representation of the distance/climb should I choose to repeat the walk, that will usually be somewhere between the two previous sets of values. Yesterdays route planned at 12.8km & 351m ascent, GPS reported 13.6/413, imported and converted from track to route (reduces datapoints) 13km/350m. In practise I suspect datapoint reduction impacts the ascent figure dispropotionately.

Rob Hindle, Sheffield

A. You don't say how much 'a lot' is - or give any indication as to how you determine how far you have actually walked - so it is difficult to tell whether your GPS is functioning correctly or not.

However, see How accurate is a GPS at measuring how long a walk is? for an answer to a similar question.

For more general information on GPS and GPS Devices - see

For fellow user Questions and Answers about GPS and GPS Devices - see