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March 14, 2013

Why Doesn’t My GPS Device Match the Race’s Mile Markers?

Written by Dena Evans

mile_markerMany runners have begun to enjoy the luxury of GPS devices measuring their daily runs.  Frankly, many runners have become so reliant on these measurements that success or failure is defined at least as much by the watch readout as how the run or workout feels.

 

Those who use a GPS device on a daily basis are naturally inclined to strap them on for race day, only to be quite annoyed by the discrepancy between the race’s official measurements and what the GPS device indicates.  Technology advances year after year, and when paying several hundred dollars for a gadget, it is a let down when the measurement appears to veer so widely off the mark.   Alternately, we may place blame on the race management, assuming a poorly measured course or sloppy monitoring.  More likely than either scenario, the discrepancy probably occurs due to the different ways in which the watch and the race record your distance traveled.

 

GPS devices measure the time it takes to receive signals traveling at the speed of light from multiple satellites orbiting the earth.  A couple dozen of these are currently in the skies, and they are arrayed so that at any one time, four or more are visible to any point on earth.  The watch essentially then builds a three or four way Venn diagram by overlapping the readings taken by each to confirm a fairly accurate location.   This is called triangulation.  Still, under a clear sky in the middle of a desert, an accuracy of a few meters either way is probably the best possible result.

 

Most importantly, GPS does not measure the distance you travel in a continuous fashion.  It take readings at different points along your route every few seconds, again maybe varying in a radius of 3 meters to 10 meters or 30 feet to each side.  What was a straight path for your actual travel, may be a fairly zig-zag line of plotted points as read by your GPS device.   Add in periodic blockages due to overhanging trees, tall buildings, and even loud noises (yes!) and you will begin to see how your watch’s measurements are a helpful guide, but by no means a perfect representation of the actual route you traveled.

 

Certainly many low-key races may not seek or receive certification by the sport’s domestic governing body, USA Track & Field (USATF).  However, most worth doing have received this certification, and certainly all of the big ones.  When the question of GPS discrepancy was posed to the “dean” of Northern California course certifiers, Tom Knight, he encouraged reading the self-termed “generic response” Doug Thurston, the Director of Operations for the Competitor Group (Rock ‘n’ Roll series, Carlsbad 5000, etc) has developed after countless inquiries on the subject.

 

Thurston’s main points are summarized briefly as follows:

  1. Courses are measured by using the shortest possible route (tangents) to ensure that when the distance is reached, no one could have run less.  Of course, that means in a crowded race, everyone is pretty much guaranteed to run at least a bit longer than this minimum measurement.
  2. ALL certified courses (ALL) include a “short course prevention” cushion, of about 5 feet per 5K or 1/10 of 1% of the distance.  For a marathon, that is about 135 feet, and is evenly distributed throughout the race, not placed at the end or the beginning.
  3. Courses are measured with a calibrated counter on the wheel of a bicycle, called a Jones-Oerth counter.  It is calibrated using a 1000’ steel tape and registers as frequently as once every three inches.

 

In short, anything that measures in a zig zag pattern is going to differ from a method that takes the shortest possible straight line between two points.  GPS devices are great tools that have allowed us to understand our daily running in a very useful quantitative way.   Course certifiers have a specific charter, method, and rigid canon of regulations to follow, their only goal being to provide the most accurate measurement possible under the ground rules.  Although we’d all enjoy if our watches marked exactly 5000 meters when we hit the finish line of a 5K, the fact that it often reads more or less shouldn’t steal any of our joy – either way, you’ll want to come back and improve your time on that course next year!

 

 

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