GPS PHENOMENON

During the past year, I have received a number of emails from runners who have participated in a few races I have directed complaining that the course was "too long." Even though the courses were certified by an experienced USATF certifier, the participants are adamant they are right and we are wrong. Now, I would never say as race directors we are always right, but if an experienced USATF certifier measured my course, I believe 100% the distance is accurately measured and the distance is what we have advertised it to be.

Some courses have a lot of turns. Most of the time, when someone who wore a GPS device claims the course was long, the course they are referring to had many turns. It is almost impossible for someone to run the course exactly the same way the course was measured, especially if there are a lot of runners in the race. Unless you are running alone and there are no other runners near you in the race, most of the time you can't always run the tangents the way the course was measured. Thus, you are actually running a longer distance. But that doesn't mean the actual measured course is long. Additionally, sometimes runners begin their measuring from the spot where they are lined up at the start, which is not always exactly on the starting line itself.

Interestingly, some GPS wearing runners have also claimed that a certified course could actually be a little short. Perhaps their readings were off as a result of losing contact with a satellite because of trees or tall buildings blocking the signal for a short period of time.

So, whereas the phenomenon of GPS technology has added some unique, valuable and fun aspects to running, don't let participants in your race fool you by saying your course is not accurate because their GPS indicated something different…these devices are not always right either.

Editor’s note: We contacted Jim Gerweck, who is Vice Chair of USATF’s Road Running Technical Council, who provides a more technical explanation below.

Even when the course is correct (and it is, almost every time), a runner wearing a GPS unit stands a good chance of having the wrong distance shown on his/her device. There are many reasons this can happen.

The most likely reason is that the GPS tracking was not started precisely at the start line, and turned off precisely at the finish line. It is hard to turn on/off your stopwatch and GPS at the same time. So, you could easily have a 50' variance from the official Start and Finish lines. That really isn't that much, and may not register. Another possible reason - GPS inaccuracy. Consumer-grade GPS units (the type most of us use, and what all "runner's" GPS units are) are inaccurate by design. The government does not allow 1-foot precision in consumer-grade units. That is why most units will show accuracy to 10 feet, or so.

Another aspect of accuracy is how often the unit “decides” where it is. The eTrex Legend HCx unit that I use for elevation profiles will only check its location every 100th of a mile (52 feet). If a runner's GPS doesn't check any more often than that, any sharp turn may not register at the apex, and yield a shorter course. However, most runners’ complaints are about a course being too long. How does that happen?

First, remember that the measurer measured the shortest possible route? They didn't have to avoid other runners and could make a beeline from one curve to the next. If you didn't run the same line, you ran further than the certified distance. That can be up to 1% of the length of a race, if the race is run on winding roads. (I measured a 5k course in a city park. I then measured the course I observed many runners using, which was wide on the curves. Basically, they ran down the center of the road. They ran about 160 feet further than the course was measured.) It is easy to find a course is "too long" when you don't run the shortest possible route.

Second, obstructions of the satellite signals. While a tree-lined course is nice to run on, it does not allow a GPS unit to keep in constant contact with the satellites. Then, the GPS unit has to “guess” where it is. Or, buildings will deflect and reflect satellite signals, “confusing” the GPS unit. My GPS unit will sometimes show me an entire block away from where I am, when I am riding between buildings. Reflected/deflected GPS signals are very common, and may explain over half of the incorrect distance.

So, you see, between not running the shortest possible route, tree cover, or signal distortion, there are many reasons why someone may get an inaccurate distance on their GPS when they are running on a certified course. GPS devices are great aids to our running, but they are not necessarily accurate when it comes to measuring distance. Many times they are. But, there are many ways they can be different than the certified course distance.  /Jim Gerweck/