In GPS We Trust

found this interesting from the race director of a Boston Qualifying Race:



In GPS We Trust

I want to address an issue on which I usually get one or two questions a year. Sometimes people think that the course is a tad long on their GPS. Personally I had always felt that the course was a tad long--maybe 26.33 miles on my Garmin. The first year of the race before we were certified when I measured it myself on my Garmin we used a slightly different (slightly shorter) course. A group of about 8 runners from the running club that year checked it on their Garmins and their average came out exactly to the 26.2 that I had measured (but some were a tad higher and some a tad lower).


However, once we had the course certified, the current course is the distance as it was certified by the independent measurer for certification. In speaking (arguing) with him about this when he first set it, he told me that their method of measuring with a calibrated bicycle wheel measure is more accurate than GPS. I never really believed him until I started riding it on my bike. And I have to say that when I ride the course on my bike it comes out exactly 26.2 on my bike computer (based on wheel revolutions not GPS).

Since then I have gone to two different race director conferences where they have discussed this issue and said that GPS is not as accurate as the certification method. Apparently this issue is not just our race but a not uncommon issue raised by runners in many races. GPS measure often seems to come out a tad longer than the calibrated wheel measure. This is an article addressing the issue that they gave us at one of the conferences, titled "In GPS We Trust", that might be of interest to some of you who may have experienced this phenomenon.