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Verifying the Keevil 10k (SEAA05/112)after a challenge by GPS wearers.

I never thought the Keevil 10k, near Trowbridge in Wiltshire, which I measured in 2005 with the Jones counter and my calibrated bicycle would ever prove to be at all controversial. Two full laps of the airfield perimeter road, with a short diversion down a runway came exactly to 10,000 metres, and the almost flat course would give opportunities for plenty of PBs. However it was not to be, for the runners wearing GPS gadgets thought they knew better, and after the 2007 event they gave the organiser a lot of hassle by publishing in the newsletters of local clubs claims based upon their GPS readings that the course was long by about 150m, implying that all those who had just failed to do a PB could have been cheated by a poorly measured or laid out course.

When race organiser Will Scott reported this to me, my immediate reaction was, "Oh dear yet another case of runners believing what their GPS says rather than the accurate certified measurement by a measurer with his bicycle calibrated and operated in accordance with IAAF procedures." See my posting on the course measurers' bulletin board.

We challenged local GPS wearers to come out and witness a remeasurement being done on 5 April 2008, and perhaps even have a Jones counter put on their bike and join the measurement ride. Unfortunately none of them turned up, although we were pleased to welcome Ian Trussler, a runner who was interested in finding out about learning about becoming an officially accredited course measurer. The official remeasurers were Stewart Little from Dorset and myself from Oxfordshire. We noted that the position of the turn round point had been clearly marked with red paint by Will. Our results for one lap were as follows

Mike Sandford (UKA Grade 1/IAAF grade A) 4997.5 m - within permissible range
Stewart Little (UKA Grade 1 measurer) 5001.4 m - within permissible range

Ian Trussler (trainee on first ever measurement ride) 5012.7 m - reasonable accuracy for trainee

All these results include the SCPF of 0.1%. The permissible range for what should have been exactly a 5000 m lap, based on my October 2005 measurement (+/- 0.1%) would be 4995 to 5005 m. No change to the course as certified in 2005 is necessary. Download a .pdf file containing my calculations for the remeasurement.

So the original official measurement was correct. The race organiser knew where to position the turn around cone correctly. The complaining runners wrongly believed their GPS reading.

Why may the GPS be 160m out in 10 km? There are many possible contributions to GPS error, including such obvious things as the runner not running along the shortest possible route and starting and stopping his GPS exactly at the start/finish line.

NEW OCTOBER 2008 - 26 Runners' GPS distances from Run to the Beat Half Marathon

Here is a list of the most useful technical links about GPS accuracy for runners which I can find.

GPS - Jones Counter comparison by surveyors in Australia (differential GPS on a bike not runners' wearing a GPS and still not accurate enough!)

Here is rather mixed bag of other links about measuring trail races with a GPS:

A trail runners blog

Recreational GPS compared with professional GPS used by surveyors

What is the difference between Consumer and Mapping/GIS GPS Receivers?


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