GPS issues explained!


"The course was too long".  "The course was too short".  We hear that more and more, lately, from runners wearing GPS units.  Trouble is, they are relying on a measuring device that is, by design, inaccurate.

Send me your GPX file! (GPX is a universal GPS format). Download the file from your GPS, and send it to

Let's start with a little review of how a course is certified.  A measurer uses a counter attached to their bike wheel.  This counter is calibrated on a known calibration course before, and after, each course measurement.  The measurer rides the shortest possible route while remaining on the race course.  The course is measured at least twice, to make sure they get the same measurement both times.  If it is off, they know they made a mistake, and have to measure again.  This is all to assure everyone that the course is the certified length.  Unless the same error was made during both rides, the course is going to be correct.

Let's now assume that the course was measured correctly, and is the correct length.  If the course was laid out properly (the measurer usually doesn't lay out the course on race day, but has supplied a detailed map to the race director, so the race staff can properly mark the course), and the intermediate marks (mile marks, usually) are located properly, the course is correct.

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.  Ah, 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 measurer measured.  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 as I observed many runners running, 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. 

Don't chastise the race director, unless you have looked at the certification map, and the race didn't follow that route.  If the course was set out properly, you ran the certified course.  The discrepancy in your measurement and the advertised distance could be your fault, or beyond anyone's control (GPS inaccuracy).