How accurate is a GPS at measuring how long a walk is?

Q. I followed the route prescribed in go4awalk.com walk ny101 The Yorkshire Three Peaks using your GPS Waypoints marked on the route and according to my Garmin I walked 25.3miles rather than the 23.5 miles shown.

As far as I am aware I did not venture off the route and found the waypoints to be very accurate within a few metres.

Could this be a problem with my Garmin or are you underselling the distance. (it certainly felt ever inch of whatever distance at the end).

Thanks for the route which I would not have even thought of attempting without the detail given. Have followed several walks in Derbyshire and am looking to widen my experiences in to the Lake District in the autumn with the help of your routes.

Thanks

Adrian Ponting, Lincoln

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A. I too have experienced this phenomenon - with walks being clocked on my GPS (an Etrex) as 25 or 26km and then only measuring 23 or 24 km when mapped.

Since I have never experienced a GPS Device-measured-walk to come in less than its mapped length, I can only assume it is a mixture of:

• the true distance covered being a bit further due to climbing, descending or walking across uneven ground (though some digital mapping products do try to take this into account)
• the fact that even on what looks like a relatively straight footpath, one wanders a bit - and
• the movement of the satellites throughout the day (if you take a reading on a rock at the start of the walk and take a second reading on the same rock at the end of the walk - the two readings will differ - by as much as 10m in some cases)

I don't think its a problem with your GPS Device and even if we are under-estimating the total distance by a little - we are consistent across all our walks.

So you can be sure that when you have completed one - you will know what to expect on your next one.

If anyone else has any ideas or undertaken any research we'd be interested to hear from them.

Hope this helps

Mike (Editor)

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A. My brother and I have different models of Etrex (mine is Legend). We went for a walk the other day and set our tripometer to zero at the same time. We both held the GPS in our hand the whole way round. The distance walked varied on the two GPSs throughout the walk, sometimes mine saying up to a third of a mile further and sometimes my brother's closing the gap. We finished a quarter of a mile different over a 6.75 mile walk.

Kind regards.

Tony Evans, Shrewsbury

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A. This phenomenon is caused by "pinning" - which is what happens when your GPS strays a bit due to Selective Availability. It is most evident when standing still or walking. What's happening is your GPS *thinks* you've moved a short distance, even though you didn't really move, and it's counting this movement in your total distance. Search for Selective Availability for a more technical and complete answer about pinning.

Mike S, Austin Tx

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