Is Your GPS Wrong or is the Course Long?
13.4? 26.6? Most of the time – you’re not crazy!
If you’ve ever run a race, it’s likely happened to you. You’re having a great race, the miles are clicking off nicely, you cross the finish line (maybe even with a shiny new PR), you look down and… your watch doesn’t read the correct distance! It’s a letdown for sure, but you don’t have to let it ruin your day. Many factors can be involved in discrepancies between your GPS and the course distance, and luckily there may be a way for you to get those two numbers closer together!
I love being a pacer for races, and a couple of times I didn’t plan for the course to be a little over and ended up sprinting the last mile to get people in at the proper time! Now if I’m pacing a half marathon, I ALWAYS bank on the course being 13.4 miles and adjust the pace accordingly. Much better success! Keep that in mind when you’re trying to hit a time goal!
Why doesn’t my watch match the course distance?
There are so many reasons that your watch may not match the course. First of all, GPS technology isn’t perfect. The signals that your watch receives can be blocked by tall buildings and trees, as well as dense foliage. GPS devices also don’t check position constantly- more modern watches have the capacity to ping your distance every second, while some are set to ping every 20-40 seconds, sometimes even longer. Then, combine both of those points– if the satellite connection is lost in between pings, data can be lost.
Beyond GPS, sometimes course markers and cones are placed incorrectly or even vandalized. Also, when courses are measured, they typically err on the side of longer than shorter and there’s a short course prevention factor of 0.1% built in.
Finally, most people don’t run the tangents in a race, which is how courses are measured out for final distance. So…
What is a tangent?
When courses are measured for events through the USATF, it’s done using the SPR- the shortest possible route. Using the SPR means that when there’s a curve, it’s measured using using tangents. If you remember your high school geometry, you’ll remember that a tangent is simply a straight line from a point on a curve. To minimize the distance, you’ll run a straight line from curve to curve. Here’s a quick diagram to help:
A- Let’s think of this line as the hypothetical course. It’s measured right along the curb and follows closely around the turns.
B- This is more likely to be your race scenario. You run around groups of people, maybe you cross the street for a less crowded aid station- you get the idea.
C- This is an actual tangent. No matter how many curves there may be, the course stays completely straight along those curves.
(Original line drawing credit to USATF Course Measurement & Management Manual, 2013. Fancy blue line drawing is all my own.)
How can I minimize the distance run during a race?
This one’s pretty basic- run in straight lines whenever you can! As the course curves, follow the straightest path from curve to curve and you will shave a couple of feet off. Over a course with a lot of turns, that distance will definitely add up! If you’re really detail oriented, you can look up the course description early, check out where the turns are, and plan ahead for those curves.
Now as a quick aside- please, please do NOT run outside of the cones to run your tangents. It’s very unsafe and may even get you pulled off the course! Additionally, in some cases, running the tangents may mean that you are cutting across the field, so be respectful and mindful and don’t ruin someone else’s day over a couple of feet.