Dear Race Director

Sage Advice from a Wise-a** Source

In this week’s advice column, we have a question from a runner curious about course measurements and GPS.  The names and locations have been changed to secure anonymity.

Dear Race Director: My name is Yash Apple and I run for the local Furniture City Espresso Racers.  I recently did a trail half marathon at Northeast Park. The race director of this race is handsome, smart, extremely funny, and an all-together great guy. But I am getting off topic. My problem is that my GPS watch did not finish at 13.1 miles and my friend’s GPS was completely different than mine.  What gives?

Dear YA:  I happen to know this super amazing, downright sexy Race Director and I also know how he measures his courses, so I do have an answer for you.

Trail race distances are always hard to measure, and especially difficult to get a consistent distance on the same course. This particular course was measured using a wheel. Essentially every foot traveled increases the counter on the wheel.  After 5280 feet you have your first mile, and so on. There are some small corrections that need to be made for this method. A lot of roots on a course may throw the distance off if the person measuring isn’t careful. Also, if the person runs with the wheel, or doesn’t keep a fixed line, it will add feet to the overall distance. This is still the most accurate way of measuring a trail run course.

The other option would be with GPS. But there are a lot of things that can change the accuracy of a GPS. A short list of factors that affect GPS:

Number and position of satellites.  This is obviously one out of the race director’s control. The more available and better-positioned satellites will get a more accurate read than less or poorly-position satellites.

Quality of device:  While it isn’t exactly you-pay-for-what-you-get, not all GPS devices are made equal, and not all devices are made for the same purpose. A military-grade handheld GPS device is going to get a more accurate read than your phone or Garmin watch. A consumer quality GPS will get accurate within about 20-40 feet, but when you are moving around (especially not in straight lines, like on a trail) over long distances the inaccuracy adds up. A higher quality GPS will also be able to connect to satellites from other entities (other country’s satellites, other corporate satellites, etc.)

Celestial bodies:  Most notably the sun and its magnetic fields can throw a GPS off much more than you might think.

Weather: Clouds, rain, snow, sun, etc will all give you different reads. Try it sometime – go out and do a five mile run on a cloudy rainy day and then try the same route again on a sunny day, see how different your GPS file looks.

Terrain and Tree Cover:  GPS, while it has gotten better, does not do great with accuracy when it comes to changes in altitude. If you are on a hilly course, your GPS will not read as accurately. An even larger factor will be the foliage.  If an RD were to use a GPS to measure a course in the winter when there is not much growth on the trees, and then puts the race on in midsummer, the resulting GPS-measured distances will be completely different.

Battery life: Low battery can also weaken the strength of a GPS.

Cellular Coverage: A phone will often use not only GPS to triangulate your location, but also get some help from cell phone towers.

This is not a full list. There are a lot of other factors that can affect GPS accuracy.  GPS tools are great, but they are not going to be exactly accurate, even to themselves.  

Example: Laurel Bluff Trail in Greensboro, NC

  • course: distance – 2.9 milesScreenshot 2016-03-07 17.30.59
  • GPS with Garmin: distance – 3.23 miles      laurel bluff garmin gps
  • GPS with Android Phone: distance – 3.14 miles laurel bluff android gps
  • Measuring Wheel: distance – 3.49 miles


Next time we write about course measurements, we will talk about what goes into measuring a road running course and why, most of the time, your GPS will come out longer than the course itself.

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