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Results from 26 Runners' GPS units at the Run to the Beat Half Marathon

I was interested to read a discussion between runners of a half marathon held on 5 October 2008. 26 runners runners posted their GPS readings claiming the course was long. However we know the course was measured by Hugh Jones the IAAF/AIMS adminstrator for course measurement in English speaking Europe and Africa, so we know that it will be the proper half marathon distance ie 13.1091 miles+ 0.0131(Short Course Prevention Factor) +/- 0.0131(maximum error for measurments with a calibrated bicycle and a Jones Counter).

So the longest the course could be would be 13.1353 miles or 42 m longer than the nominal 21.0975 km for the half marathon. n.b. measurers include the SCPF to ensure the course will not be less than the advertised distance.

Here is a plot I have made from the 26 runners who posted their GPS results:

Runners' GPS measurmentsThe average distance measured was 13.39 miles - ( 0.26 miles or 2.2% long), the results are clustered around this average with a standard deviation of 0.11 miles.( or 0.8%)

Compared with course measurers who would typically get results clustering around the true distance with a standard deviation of about 0.004 miles ( 0.03% ) we can see that the scatter of the GPS is about 25 times greater than the measurers' calibrated bicycles. But in fact more worrying than the scatter is the bias towards long measurement values of 2.2% which is about 50 times worse than the typical bias for a group of calibrated bicycles (which is actually derived from the accuracy of the precision steel tape they use to measure their calibration courses and other sources.

Converting the above numbers to metric distances: course measurers would get 21097m + 21m (ie SCPF) with a standard deviation of 7m and a bias of about +/- 8m. Whereas the runners' GPS gave a standard deviation of 180m and a bias of + 460 m.

We can conclude that these runners' GPS results were 25 to 50 times worse than the accuracy for an official course measurer.

What are the reasons for the poor GPS performance?

  1. Not starting and stopping the GPS at the course start and finish
  2. Not following the Shortest Possible Route
  3. Loss of satellite visibilty under trees and in the shadow of buildings
  4. Imperfect algorithms in consumer type GPS units worn by runners.
  5. Organiser laying out course longer than that measurered by the course measurer

Most runners concentrated on point 5 - blaming the organiser. However experience elsewhere shows that runners' GPS nearly always give long results eg my write up of the Keevil 10k investigation. Normally runners can check whether the course follows that the course measurer's route by refering to the maps that are published on this website for all races in the South of England that have a Certificate.of Course Accuracy issued by me. However, in this case although I got a measurement report from Hugh Jones and it checked out OK, the organisers of the race decided not to proceed with obtaining a UKA road licence so they can not be issued with a UKA Certificate of Course Accuracy. Therefore the the services of this website for the publication of the course map are not available for this race at present.

©Mike Sandford - 25 October 2008


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