April-June issue of Distance Running, AIMS’ quarterly magazine, has a nice opinion piece by Hugh Jones, about GPS. See below.
If you wish to download the whole magazine, you can do so at:
Good article. Thanks Pete. I was especially happy to see that he stressed people not running the SPR as the primary cause of the long readings, not inaccuracy of the GPS itself.
At one point it is mentioned that extra movement of the wrist may cause long measurements. I think if this is true at all, it has a very minor effect. Many people imagine the wrist as moving back and forth as you run, which it does relative to your center of mass. But relative to the ground it is always moving forward, just not at a constant rate. Any extra distance would come from vertical or sideways motion, and I think these are very small.
Mark, not sure if I agree w/ that. I believe Norrie Williamson did a test of GPS units where he wore the unit on his arm and then clipped to his hat, and the latter was more accurate due to less extraneous motion.
BTW, I had a Garmin direct me down a non-existent side street a few weeks ago.
I haven't seen his results. Are they posted somewhere?
I've done several different experiments wearing a GPS on my wrist, both while running and while biking, and I've seen no significant difference between the two types of measurements.
I think it was in an old issue of Measurement News. I'll see if I can find it.
It was in MN Issue 115.
Download it at:
The accuracy of commercially available GPS devices has improved dramatically since that article was written in 2002.
I will agree with Mark. As GPS units generally record their location only once every 100th of a mile (52 feet), or every second (don't know if you would have enough memory to record an entire marathon with this setting), the motion of the hand has nothing to do with the overall distance, especially since, as Mark said, the hand is always moving forward. The article said the side-to-side motion at a water station may make an impact, but I would argue it is not registered.
I also dispute that the overall distance for a marathon could be up to 84 meters long (don't they know how to spell over there???). The .1% SCPF should mean that the MOST a course may be is 42.1 meters long. It should be shorter than that, as the bike wobble will eat up some of the SCPF. There is no way to make it longer than 42.1 meters long, if calibration was done properly. I also take issue with the assertion that a steel tape is not accurate, since we take temperature into account, and put specific force on the tape.
But, overall, I think this is a very good article, which every runner who wears a GPS should read.
I basically just wish all the GPS-wearing runners would realize their tech-toys are just guides to running, not the ultimate authority in course length (that would be us, of course!)
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