Have you ever finished a race and found that your GPS watch was telling you that you ran further than the race distance? Before you write a nasty email to the race director accusing him or her of mismeasuring the course, consider whether your GPS might not have been telling you the truth.
USATF-certified race courses are measured multiple times using a measuring wheel attached to a bicycle. These measurements aim to track the straightest line possible, which ensures that the approved course is, at the very least, no less than the correct distance.
The problem, though, is that the course measured by your GPS may not take the same approach. Your GPS relies on satellite tracking, but this tracking doesn’t follow every step of your route the way a measuring wheel does; instead, it uses a series of plotted dots that locate you within about a 10 foot range of where you’re actually standing. Those variations can add up to a significant difference from the plotted route over the course of a 10K or marathon.
The way you run a particular race can also affect the measurement. If you take wide turns to avoid crowds or obstacles, or veer towards aid stations or port-a-jons, that can add distance to the straightest-possible-line measurement. Also, the accuracy of many GPS watches are affected by tree cover, leaving their readings less reliable.
The bottom line? Use your GPS in training and racing to get a good idea of your distance and speed, but don’t accept those numbers as the gospel truth.