Almost every race director has heard the complaint, oftentimes dozens of them, immediately after a running race is finished.
"My GPS watch," the complaint goes, "says that your course was too long."
A half-marathon often comes out to 13.3 miles on the GPS. A 10K comes out at 6.3 miles. A marathon shows 26.5 miles, making the last stretch agonizing because, well, wasn't I already supposed to cross the finish line?
Why would they make the course longer than the advertised distance?
"We expect a little bit of (questioning) every year," says Rusty Snow, race director of the Santa Barbara Marathon.
The truth, in all likelihood, is that the race organizers didn't make it longer. The GPS watch did. While GPS is a great tool for runners, it's not going to be more accurate than the measurement standards in place on a USATF-certified course (not all races are USATF certified). The certification process is too precise, with numerous safeguards in place to prevent an error.
The complaints roll in, though, to the point that directors of major races have generic emails with a thorough explanation ready to send back. There's a lengthy presentation floating around the Internet called "In GPS We Trust" that spells out the exact reason why your GPS watch might be a little off.
The USATF Process
USA Track & Field, the governing body that certifies a course distance, has a 66-page manual that details the proper way to measure and certify a race course.
In short, the go-to tool is a Jones Counter, which attaches to a bike and determines distance proportionate to the revolutions of a bike wheel. Race organizers can attach the Jones Counter to their bike, and then test it no fewer than four times on a "calibration course" using a steel tape and a flat, straight section of road. A thermometer is also required, because the outside temperature can have a minor impact on the length of the steel tape.