go to course measurement home pageLimitations of GPS for Road Races

Some views from non-course measurers

When the editor of Athletics Weekly characterises course measurers as "a bloke on a bicycle", completely overlooking our excellent lady measurers, and then calls for our replacement, mentioning "GPS devices and hi-tech computer technology" as alternatives, one hardly knows whether to laugh or cry.

So what are other running experts who are non-measurers saying about GPS?

Ian Williams the developer of the fetcheveryone.com website

Ian wrote a very good blog  for the Guardian ( link ) with an excellent simplified explanation of how gps works and what goes wrong. He includes some good advice for runners. Well worth a read for these tips. Here is his opening:

Course accuracy: why your GPS watch isn't infallible

‘The course was long!’ ‘I ran an extra mile!’ – we’ve all heard it. But why do people always assume that their GPS watches are dead accurate? There’s something that is guaranteed to come up on every finish line of every race. No, not vomit – but course accuracy. These days, lots of runners have GPS watches, and it’s easy to imagine that they offer infallible evidence that the course measurer should be made to give you a piggyback to the finish line......more

Even Garmin, the long time manufacturer of runners' GPS watches, warns be careful ........

Why is my GPS device displaying incorrect distance/speed readings?

...........GPS should be utilized as a guide, not as the absolute definition of what happened. This applies to any device that is not a professional survey tool. See more

Why does my GPS device appear to be moving and/or gaining distance while I am stopped?

GPS drift is caused by many external factors and consumer grade GPS devices cannot account for these. GPS drift causes the device to appear to be moving on its own. It is most obvious when looking at a track or when zoomed in all the way while at a standstill.

Satellites send their signal through the atmosphere down to earth. The atmosphere distorts this signal, and other environmental factors (such as trees, hills, mountains, buildings, cars, reflective surfaces and more) can further degrade the signal.

In the past, a satellite signal weakened by the environment caused a loss of position. High sensitivity chips were created so that it was no longer a question of if you had a position lock, but how accurate that position lock would be.

Now only the weakest signal prevents the device from locating your general position, but as a result, the decrease in accuracy that is introduced by the environment causes GPS drift. In other words, the device is more sensitive than the environment allows it to be accurate.

Being mindful of the limitations of consumer GPS devices will help alleviate concerns regarding their accuracy. See more

Finally a view from a top measurer:

So what do measurers say about all this? We have had a section on this website with results from GPS for many years. But why not listen to how it was recently summarised by our chairman Hugh Jones, who is also one of 4 AIMS/IAAF International Course Measurement  Administrators,  and very active measurer both in London and around the world. He reminds us of his experience, "Way back in 2003 (admittedly the technology was probably not so good at that time) I found that what got GPS-recorded as a mile through Canary Wharf (at that time we used South Colonnade) was approximately 1200m as we old blokes on bikes would measure it"