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.