In this past year I’ve had a Garmin GPS watch, I’ve run about a dozen races while wearing it.  In every race, without fail, it’s always the same: the end distance on the watch is always longer than the race distance.  The Broad Street Run 10 Miler may end up as 10.12 miles, and a marathon may show up as 26.44 miles.  The additional one or two tenths of a mile may not seem like much, but they matter when you’re trying for a particular time goal or if you’re saving your energy for what you believe is the final distance only to find out that the race is not over.  And, of course, the GPS watch never measures the distance as less than the race distance.

Is it the watch, the course or something else?  After being stumped, it turns out that the answer is it’s the way the course is measured combined with slightly less than completely efficient running.

Because GPS watches depend on satellites, the watches may not always be completely accurate, though I think they’re pretty close.  I use Google Maps to measure a running route, and the watch reading is often right on the money.  When it comes time for a race, though, the distance measured has always been off.  During this year’s Broad Street Run, which, save for rounding City Hall, is almost as straight as a race can be, other competitors and I noted that our watch beeps marking every mile kept alerting before the course mile markers.

The primary explanation, I recently learned, is from the way courses are measured.  USA Track and Field certifies course distances.  According to Rule 240, courses shall be measured by the runner’s shortest possible legal route.  So, even though you may stay in the middle of the road around curves, USATF measures with a line that would cut the turns by the curb pretty close.  It’s also unlikely a runner will race in a completely efficient manner.  Each time you veer off the straightest line, such as to get water or avoid other runners, results in additional distance from the “shortest possible legal route.”  Certainly, during Broad Street, I took a wide turn around City Hall and also veered to both sides of the road to greet spectators.  Additionally, to ensure courses are not measured short, USATF requires an addition of 1/1000th the stated distance.

Given these factors, it’s not surprising that GPS watch measurements are off and always with a longer distance.