Recently while watching the Run856 chatter on Facebook we saw a great deal of back and forth about just about everyone being slightly off when looking at their watch time and distance versus their race chip time and the known and verified race distance. One of the Run856 users shared this explanation from the USATF:
Taken directly from the USATF below:
USATF/RRTC Position on GPS used by runners
Effective June 6, 2014
GPS devices work by receiving signals from satellites. The quality of different GPS units can vary, but all of them can be affected by conditions such as buildings in urban environments or heavy overhead tree cover that interfere with reception of the satellite signals and can cause them to be inaccurate.
Race courses Certified by USATF are measured by a proven method that incorporates the calibration of measuring devices against a steel tape and are verified by multiple measurements.
Race courses are measured along a well-defined path called the “SPR”—the Shortest Possible Route that a runner can possibly run. Most runners don’t actually run the SPR, so the distance recorded by their GPS device will usually be longer than the certified length of the course, even though the course was properly measured along the SPR according to USATF rules.
More articles that go a bit more in depth on the topic:
Competitor Running: Is Your GPS Watch Lying To You On Race Day?
Runners World Magazine: chip time vs. gun time vs. GPS time