What is up with my GPS...

In the mail this week, I received my magazine from the Atlanta Track Club.  I usually leaf through it to see if I see anyone I know in the pictures and then give it a toss.  This month, it had an article about GPS devices.  Specifically, the complaint that you hear all the time about a course being "long".  You have seen it in a 1000 blog entries and facebook updates.  You know the ones...  "Since when is a 10k 6.4 miles?" or "I would have PRd but the course was 26.8 miles".  I *think* we have probably seen this enough to know that the problem was not with the course but with the runner and their GPS device.  (Assuming the course is  USATF certified).  What was interesting to me was that this article went into a lot of detail about how the course is measured in order to get its USATF certification.  (I'll give you the cliff notes version...)

 - The methodology for measuring a course is outlined in a 100-ish page manual issued by USATF.  Basically, the course is measured with a Jones Counter device attached to a bicycle.  The bicycle is then ridden over the course a minimum of four times to calibrate the bike/device.

 - After the device is calibrated, then they ride the course a minimum of two more times one foot away from the curb or road edge using the shortest possible route.  Turns and corners are taken as closely as possible and tangents are taken as straight as possible. (so if you want to run the exact route, stick to the inside curb...)  :)

 - Your GPS needs a clear signal from three satellites in order to operate.  Clouds, trees, buildings and even people can interfere with the ability to triangulate your position. The coordinates are generally within 10 feet of your device which can add up over the course of a race.  In addition to the satellite issues, running itself makes it impossible for the GPS to be perfectly accurate.  Running around other participants, over to an aid station or taking wide turns all add extra distance.

...And there you have it.  The next time your course measures a little long, you can thank your GPS.