Why Doesn't My GPS Watch Match the Course Measurements

Nathan Porch

Friday, June 19, 2015

The following article, which appeared in Road Race Management Newsletter, helps explain why sometimes a GPS might not agreee perfectly with the calibrated and verfied course.

Among the phone calls and emails a race director inevitably receives from runners after an event complaining about stale bagels, long bathroom lines and T-shirt shortages are sure to be a few (and sometimes a lot) claiming that your course was the wrong length. Usually, they state that the course was too long (funny how no one ever seems to complain when a course is short, isn’t it?), and some will go so far as to request/demand that their time be adjusted, particularly in the case of a marathon when a Boston qualifier is on the line.

Virtually every one of these will come from runners who wore a GPS device during the race and found out the distance was anywhere from a few hundredths to several tenths of a mile off.

What can a race director do with these claims, aside from hitting the “Delete” button on your email? Well, there are several reasons you can provide that should convince all but the most irrational runner that, to flip Shakespeare on his head, “the fault lies not with us, but in the stars.”

More accurately, it’s in the artificial heavenly object known as satellites that are cause for much of the error. GPS units calculate distance based on triangulation of readings taken from a series of fixed orbiting units, but the degree of accuracy depends on several factors.

GPS watches typically worn by runners, costing several hundred dollars, can’t achieve the results obtained by survey- or military-grade units, which sometimes use two base units that can read the satellite signals at a higher degree of precision. Even the best commercially-available GPS unit is only accurate to about 12 feet at any given time, and can be hundreds of feet off in accuracy. Most units will indicate what their current accuracy is, and it can vary from 12 feet to 350 feet or more.

GPS units must have a clear view of at least three satellites to get a reading, and the more they can acquire, the more accurate they are. However, trees, buildings, and even a runner’s body can interrupt the signal, making it less accurate at any time.

Further, they only check their position periodically, not constantly. Some units check every second, some every 20 seconds. The user can sometimes set the unit to check at certain time or distance intervals, but if it has lost contact with the satellites, it can't tell where it is, so it misses that checkpoint. So, if someone is running quickly, they may make a few turns while the unit doesn't have contact, so that section will be measured incorrectly.

The other part of the equation is the way the runner ran the course versus the way it was measured. A certified course is measured along the Shortest Possible Route (SPR), a line that cuts all the tangents just one foot from the curb or road edge. Very few elite runners, with an unimpeded road available to them, tend to run that tightly. For those farther back in the pack, the crowd of runners around them makes this almost impossible, and possibly not worth the extra effort it would require to weave through the field to follow the SPR. Also, runners may start their watches before reaching the actual starting line and stop them after the finish.

These two factors are the primary cause for readings that don’t agree with the actual course distance. Tests performed by members of USATF’s Road Running Technical Council have found that runners usually will get a reading indicating the course is 1 percent long. (Several threads on the topic are available on the RRTC Bulletin Board at http://measure.infopop.cc/eve).

Strictly speaking, all certified courses are long, since a 0.01 percent Short Course Prevention Factor is added to ensure they don’t come up short and fail validation in case of a record, that is probably not enough to explain the longer readings obtained by runners’ GPS units. The RRTC has not fashioned an official statement on GPS measurements, save to state that they are not accurate enough for course certification, but increasing requests from race directors may lead to one being created and voted on at the next USATF Annual Meeting.

Of course, it’s crucial that you make sure your course was set up and run as certified. It’s a mistake to rely on memory when locating critical points like the start and finish, and it’s equally important to make sure runners didn’t inadvertently go off course due to poor marking or course monitors’ errors.

In short, the best response to runners claiming your course was long is to tell them their GPS unit isn’t quite as accurate as they think (something many won’t believe or want to hear), and that they probably didn’t run the course as tightly as it was measured. Stating this beforehand, on the race website on the course information page, should go a long way toward reducing those post-race calls and emails.

Oh yeah, and get fresher bagels and a few more PortaJohns, too.