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Discussion Forum - Gear ! - GPS accuracy


Posted: Wed 19th Jul 2006, 9:17
Joined: 2006
That is a fair point Geoff, I hadn't sat down and worked it out for myself. However, a large proportion of the course was flat so the extra distance due to ascents/descents may be somewhere between your two scenarios, maybe accounting for an extra mile or two but nowhere near the 4.5 miles that was measured. Overall though I would tend to believe that the GPS distance was correct, as it is supposed to measure to within about 20 feet.
Author: Geoff Saunders
Posted: Wed 19th Jul 2006, 0:28
Joined: 1972
Local Group: Merseystride
Sorry to be a clever clogs, but the amount of ascent really is a red herring. I don't know how much ascent there is in the Trailwalker Challenge, but assume 8000ft (roughly pro rata to a typical LDWA 100). If the gradients were the same throughout, both up and down, and there were NO level bits in between, the walked distance would be less than 1/10 mile more than the map distance. At the other extreme, if the whole route was level with the 8000ft concentrated into one vertical climb and descent, the incremental distance would still be only just over 3 miles. Both these scenarios are unrealistic, of course, but the first one will be much nearer to the true situation than the second. The amount of ascent makes surprisingly little difference to the walked distance. Blame Pythagoras.
Author: Chris Chorley
Posted: Tue 18th Jul 2006, 21:00
Joined: 1982
Local Group: Norfolk & Suffolk
1:25000 OS mapping, as I think it says in Guidelines for Events (LDWA Publication) or Guidelines for 100s. Used to measure off a paper map, now use Tracklogs OS mapping on CD, but it comes to the same thing, I don't think that takes account of ascent (I'll try an experiment). We did once try using a surveyor's wheel, a problem on muddy fields !
Posted: Tue 18th Jul 2006, 9:33
Joined: 2006
Thanks for the reply Peter.

I was wondering if that had been the case. That would mean that the Ghurkha teams who ran the course in 10 hours and 17 minutes are even fitter than they think! I would have thought that to be an official distance they would have had to measure it more accurately than just going by the map. How do others usually measure course distances??
Author: Peter Haslam
Posted: Mon 17th Jul 2006, 17:05
Joined: 1992
Local Group: East Lancashire
Hi Gary, you are quick off the mark, I only posted your membership pack this morning!!!!

I imagine that the route was measured on a map which is in 2D. A GPS, I think, measures in 3D, taking in all the ups and downs.
Posted: Mon 17th Jul 2006, 14:40
Joined: 2006
This weekend I completed the Trailwalker Challenge on the South Downs Way. I was informed before the event that it was 100km/62 miles, but my Garmin Legend GPS made it 67 miles. I was assured by one of the organisers afterwards that it was 62 and a half miles. As we didn't go off the track more than a few yards is my GPS likely to be giving me inaccurate information or is the course longer than they think it is??

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