Race Distances and GPS


When I participate in a race, I try to set a pace I think I can maintain and then, desperately, try to stick to it.  Often, I eventually reach the point at which I watch my GPS count down the remaining fractions of the course.  “Only ¼ of the run remains!, only ⅕ of the run remains!, only ½ mile left!!”   But, so often, my GPS says I am done while the finish line is off in the distance.  What is the problem?  Is my GPS off or was the course measured inaccurately?

GPS (global positioning system), is a technology that has been developed and maintained by the United States government since 1973.  It became fully operational in 1995.  GPS satellites fly in medium Earth orbit at an altitude of approximately 12,550 miles (20,200 km).  Each of the 24 operational satellites circle the Earth twice a day.  This picture is from www.gps.gov.


The satellites in the GPS constellation are arranged into six equally-spaced orbital planes, each of which contains four “slots”, occupied by satellites.  This arrangement ensures that users can view (use) at least four satellites from virtually any point on Earth.  Since at least three satellites are necessary for triangulation of the position of a GPS receiver, having at least four available at all times assures that a measurement is available.  The US Air Force maintains the satellites and normally flies several extra satellites to maintain coverage whenever the baseline satellites are serviced or decommissioned.  In June, 2011, there was an expansion and reconfiguration of the satellites, so that three of the extra satellites became part of the constellation baseline.  This led to improvement in GPS coverage since, now, there is effectively a 27 slot constellation.  There is a wealth of additional information about the GPS program, the different types of satellites, and anything else someone could need for a cool high school research paper on www.gps.gov. Along with the US-operated GPS program, Russia, the European Union, India, and China have been developing, or using, GPS-like systems, as well.

GPS receivers (such as in the sport devices many of us wear) all use similar technology.  GPS has been improved over the years by differential GPS (DGPS), which involves using a fixed land-based point, along with GPS satellites, to improve accuracy.   Wide area augmentation system (WAAS) is a further improvement of the use of ground-based stations to improve accuracy. Here is a detailed discussion of this technology.

The following diagram is from http://www8.garmin.com/aboutGPS/waas.html:


100 m: Accuracy of the original GPS system, which was subject to accuracy degradation under the government-imposed Selective Availability (SA) program.

15 m: Typical GPS position accuracy without SA.

3-5 m: Typical differential GPS (DGPS) position accuracy.

< 3 m: Typical WAAS position accuracy.

What this means is that, with WAAS, your Garmin or similarly equipped device can, ideally, provide accuracy with an error of under 3 meters 95% (NOT 100%) of the time.  The other 5% of the time, the error may be up to 10 meters.  However, these devices are not uniform.  Some are just better.  Here is an interesting recent review of a number of popular devices.  Aside from differences between devices, accuracy can be affected by a number of factors, including mountains, tall buildings, clouds, and interference to the radio signal.  Accuracy can also be affected by the way that the data points are recorded. GPS devices do not collect continuous data.  Instead, they collect a series of data points at regular intervals, up to every second.  If a course involves a lot of hairpin turns, or turns in areas with a lot of trees, for example, it is predictable that data points would have some “misses.”  The GPS unit, itself, or the software program to which the data is uploaded, will then connect these data points and use additional algorithms (data smoothing) to create the best estimate possible of the actual course that was covered.  Since the algorithms are not the same, different, well-respected, software programs can produce varying results.

So, GPS is not a accurate as many of us would like to believe.  But how does this relate to racing?

With regard to USA Track and Field (USATF) certified running race courses, there are very strict rules about the measurement of these courses.  Take a moment to follow the hyperlink and skim the pdf about these rules.  Even temperature is a consideration in accurate course measurement.  In the case of marathons, which are defined as 42,195 meters, the international convention, as established by the Association of International Marathons and Distance Races (AIMS), is to add 1/1000th of the distance (42.2 meters) to the race, to ensure that marathon courses are never too short. This is clearly very important when paychecks, sponsors, and running careers can be attached to race times.  Therefore, with regard to certified non-marathon-distance running races in the US (and in other countries with similar strict regulations) and to certified marathons worldwide, participants can be very confident about the distances they have run.  But GPS-based measurements are usually long, compared to certified running courses.  Why is this?

The biggest reason why GPS measurements of race distances are usually long, compared with the certified distances, is that the courses are measured using the shortest distances around corners and using straight lines from point to point.  Every time we, the tired racers, do not round a corner at the apex (and, thus, take the shortest distance around the corner), we add distance.  DC Rainmaker has an excellent post about this topic, in which he illustrates that, in a two lane road, runners add about 12 meters per turn in which they take the longest path rather than the shortest path around the turn.  This adds up quickly.  With regard to running in straight lines, even while training many of us do not run perfectly straight lines.  In a race, every time we dodge around slower runners, every time we move over to high-five spectators, and every time we deviate from a straight line to get water at an aid station, we add distance.  The end result of adding distance is that a GPS watch will often overestimate the speed you are running, since it estimates pace based on time elapsed and distance run.  If you have a set pace in mind for a race, this inaccuracy needs to be considered. Therefore, trust the mile markers and use GPS estimate as just that: an estimate.

The story with triathlons, however, is surprisingly different.   Please stay tuned for my next post.

Published February 1, 2015


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  1. Pingback: Are Iron-Distance Courses Really 140.6? Read This Before You Get That Tattoo. | An Allergist/Immunologist's Guide to Living Well

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