Yesterday Joan and I had a visit from Doug Thurston, who used to direct the Columbus Marathon before Joan took over the job. While talking of old times over dinner, Doug mentioned that he has to deal with many inquiries from GPS-equipped runners who are convinced that the course was inaccurate. He worked up what he calls a “generic reply.” It follows.
GPS Readings on Certified Rock ‘n’ Roll Marathon or Half Marathon Courses
Thank you for your comment and question about the length of the Rock ‘n’ Roll race you recently completed. Because we often receive questions about the course length compared to GPS readings, we have developed this standard response.
The course you ran was certified as accurately measured by USA Track and Field, the governing body for road running in this country. Courses are not measured by GPS, tape measure, surveying equipment, car, or by walking with a measuring wheel. All road race courses are measured by a standard bicycle fitted with a calibrated mechanical counter on the front wheel.
GPS readings on the course are almost always slightly longer than the distance measured with this mechanical counter. There are several reasons for these differences:
1) Courses are measured by the shortest-possible route available to runners on race day. This means that the measuring bicycle is ridden within a foot of the curb and turns are “straightened out” as much as possible. This method is used to ensure that no one runs shorter than the stated distance. With other runners on the course, however, this shortest route can be difficult to run exactly as measured.
2) All certified courses include a 1/10th of 1 percent (one-thousandth percent) "short course prevention factor." This is a small extra cushion to again make sure no one runs less than the stated distance. So, in other words, a certified 10 km (10,000 meters) road race is actually measured as 10,010 meters. For a full marathon, that means 42.1925 extra meters, or a little more than 135 feet. This extra cushion is spread out throughout the course, not simply added to the start and finish, and is present in ALL certified courses.
3) And, lastly, consumer GPS devices are not 100% accurate. We have found they are typically 1-to-2% off. GPS accuracy is affected by elevation, the number of turns, tree coverage, tall buildings, bridges and overpasses, and the quality and quantity of satellite reception. In a half marathon, a 1% difference is more than a tenth of a mile. So, readings of 13.25 miles or more for a 13.10938-mile certified half-marathon and 26.50 miles or more for a certified 26.21876-mile marathon are not unusual and are to be expected.
I realize this may be a long answer to a simple question, but we take course distance very seriously and all of our races are certified by experienced measurers, many with decades of experience. I am internationally-accredited and I have measured many of our courses (San Diego, San Antonio, Las Vegas, Seattle, and New Orleans, for example). When I measure a course with my mechanical counter, I also have a Garmin Edge 705 mounted on my handlebars for reference, elevation, and computer mapping. My Garmin reading is always longer than the distance measured with my mechanical counter. For example, I recently measured a half marathon, which is 13.109876 miles, and my GPS read 13.30 miles.
The GPS is a useful and informative training device. I use a Garmin when training and racing. I have come to understand and accept that the information it provides me about distance, pace-per-mile, average pace, elevation, etc., though very close, is not 100% accurate. I do trust the stopwatch function, of course, but I know the distance feature will almost always give me a reading that is “too long” when I race on a certified course. (Consequently, the "pace per mile" or "average pace" functions are too fast.)
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/). If you have any questions, contact me directly at dthurston@competitorgroup.com or by phone at 858-228-3763.
Thanks for your participation and best wishes for your training and racing.
Doug Thurston
Director, Operations
The Competitor Group
Organizers of the Rock 'N' Roll Marathon Series
9477 Waples St., Suite 150, San Diego, CA 92121
dthurston@competitorgroup.com
www.competitor.com
Office (858) 228-3763
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