It's a lengthy matter

2009-01-17 00:00

THE recent advent of fitness GPS technology has seen runners questioning and berating race organisers about accuracies of their courses.

ASA and IAAF rules require courses to be measured using the highly accurate bicycle counter method implemented worldwide.

The concept is simple ­ a digital counter is fixed to bicycle wheel that is calibrated by riding four times over a steel-tape measured 300 to 500-metre distance. Simple proportion then calculates the number of clicks from the 300m to one kilometre distance. Experience indicates this to be accurate to better than 1m/km, so this is used as a safety factor.

This one metre safety means a correctly measured marathon will be 42.195km plus the 0.042195 km safety factor, plus or minus the 0.042195km accuracy. (i.e. Between 42.195km and 42.279km).

The process takes into account the weight of the rider, air and road temperatures, tyre pressure, and even the road surface, and relies on the skill and commitment of the rider to take the shortest legal line between and around corners without shortcuts; It¹s about practice, experience and running knowledge.

With military GPS able to pinpoint buildings, roads and even people, it is understandable that runners consider their fitness devices to be similarly accurate. This is far from the truth as a series of tests have proven.

In 2002 and 2003 the first units were compared with track distances and the bicycle and used to measure the 250km Augrabies Extreme and then to validate the 1 000-mile Cape to Johannesburg Relay to Guinness Record Book standards.

The cross-calibration required between bike, vehicles and GPS, was a source of frustration to manager Nic Bester, who failed to appreciate the relative accuracy of the three. In the final analysis that record was claimed with only a 100m cushion on the requirement to prove the route.

More recently studies have used races measured by very experienced IAAF A-grade measurers, which suggests the course meets the IAAF criteria.

Analysing GPS distances taken from runners at the race finish has highlighted a consistency of GPS errors. In a UK half-marathon last year the average of 26 runners¹ distances was 21.548km (2.2% long) with a standard deviation of 117m.

Bill Reynolds, President of British Association of Road Races, and I undertook a similar study in Beirut last November, but also put two different GPS models in the lead car, which I drove around the route closely following the line measured by bicycle.

The car units recorded 42.61km and 42.60km respectively. This was amended to allow for the car width, which prevented us from getting into the corners on the racing line, to give a distance of 42.5km.

The runners¹ also averaged 42.62km and ranged from 42.39 and 42.82, with a standard deviation of 137m and an accuracy of 0.76% and 0.96% based on longest and shortest possible bike measurement.

The study highlights five sources of error: not starting and stopping the GPS at the course start and finish; not following the shortest possible route; loss of satellite visibility under trees and in the shadow of buildings; imperfect algorithms in consumer type GPS units; organisers laying out course longer than that measured by the course measurer.

Although runners tend first tend to blame the organiser, it is the other four that are most common.

Conscientious runners will eliminate the first cause leaving the middle three, which cover a myriad of problems:

* When coming through an underpass or other loss of signal the GPS ³dances² around trying to re-locate, potentially adding distance.

* Wearing your watch on one wrist puts you a metre out from the measured line when turning the other direction. Every 90º turn adds 1.5 metres and more if other runners force you wider; twisty courses exacerbate this.

* Going off line for drinks, turning towards the table, and diversions to the portaloo all add distance.

* Your wrist travels further than you run, not by much but it adds up at 1 000 times per km.

* Battery levels change the accuracy particularly where the GPS is separate to the watch.

The studies confirm that GPS normally read long.

Before berating a race organiser about his long course, remember that your GPS is probably reading long and in the good conditions expect an accuracy of 8m per road kilometre and 16m per track kilometre.

And don¹t be surprised if the referee disregards your complaint. The use of technical devices for pacing are prohibited under IAAF rule 144.2 ­ but that¹s an article for another day.

References and other linked postings:

A trail runners blog

Recreational GPS compared with professional GPS used by surveyors

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

A discussion on GPS Units in Running

Mike Sanford¹s study of 26 runners at the UK Half Marathon

GPS - Jones Counter comparison by surveyors in Australia

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