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GPS Readings on Certified Courses
If you just ran the Bakersfield Marathon or Half-Marathon and wore a GPS watch (Garmin, Timex, etc.), used a Nike + or a smart-phone mapping program, you probably found it read more than 26.2 or 13.1-miles. This is not unusual. As a matter of fact, it’s typical.
But don’t worry, the Bakersfield Marathon & Half Marathon course was not long. The course you ran was certified as accurately measured by USA Track and Field, the governing body for road running in this country. All road race courses are measured by a standard bicycle fitted with a calibrated mechanical counter on the front wheel. Courses are not measured by GPS, tape measure, surveying equipment, car, or even by walking the route with a measuring wheel.
I am an experienced course measurer as well as a runner who wears a GPS when I train and race. I have found GPS readings on certified course are almost always longer than the actual measured distance. There are a couple of important reasons for this:
1) Publicly-available GPS devices, while amazingly precise, are still not 100% accurate. After all, they are receiving signals from at least four different satellites more than 12,500 miles above the earth! I have found most GPS devices measure 1-to-2% long. Others may find their GPS on this or other courses reading 1-2% short. GPS accuracy is affected by elevation, the number and frequency of turns, tree coverage, tall buildings, bridges, tunnels, and the quality and quantity of satellite reception.
How much difference can 1-2% make? In a half marathon, 2% is more than 1,380 feet or more than a quarter mile. Therefore, readings of 13.25-13.35 miles or more for a certified 13.10938-mile half marathon are not unusual. This doesn’t mean you necessarily ran that far: it means that is the distance your GPS recorded.
2) Courses are measured by the shortest-possible route. This means that the measuring bicycle is ridden within a foot of the curb/pavement edge and turns are “straightened out” as much as possible. For most runners not at the very front or very back of the race, however, this shortest route is not always available. This is because of the thousands of other runners on the course, running to one side to access a water stop or a restroom, the desire to run alongside friends, yielding the fastest line to passing runners, etc. This is true with any road race but you would be surprised how much these small changes in your running “line” can add up over the entire distance.
The GPS is a useful and informative training device. I trust the stopwatch function, of course, but I know the distance feature will almost always give me a reading that is “too long.” Consequently, if I use the "pace per mile" or "average pace" functions, they are going to register times that are a bit too fast. For example, if your Garmin computes that you are running a 10 minute-per-mile pace, you are likely actually running 10:06-10:12 minutes per mile (the typical 1-2% variance).
If you want to learn more about how courses are measured, visit the “Course Certification” area in the “Products and Services” section of the USA Track and Field website (http://www.usatf.org/events/courses/certification/).
Note that if you used a smart-phone based measuring system (e.g., Mapmyrun on your iPhone), the measured distance is usually even less accurate than a GPS watch like a Garmin. Here is a very-comprehensive article comparing the accuracy of various devices: http://www.dcrainmaker.com/2011/06/2011-sport-device-gps-accuracy-in-depth.html
If you have any additional course measurement questions, please feel free to contact your Bakersfield Marathon Race Directors at firstname.lastname@example.org.
Thanks for your participation and best wishes for your training and racing.
USATF Official Course Certifier