Why does my GPS disagree with the course length?

gps watchTime and time again, I hear runners wonder why their GPS (either their watch or smartphone) doesn’t show the same distance as the course length. Even when I try to explain that these lower-end GPS can be off by at least 5 percent, they still wonder why their technology shows the course was either short or long. (The GPS that farmers use has sub-inch accuracy, but it also requires an RTK tower to add accuracy to the satellite signal triangulation and costs about $30,000 for a system.)

The USA Track & Field Association, which has set out specific practices to measure and certify race courses, has this response:

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.

The goal is to ensure the course is NOT short. Some certifiers add a few feet per mile as a buffer.

Should a runner set what is presumed to be a record, the course must be re-measured. If the course is short, then there is no record and the course certification will be withdrawn. If the course is long, then the record can be submitted to USATF for validation.