There’s a topic I’ve been wanting to cover for a while now. Even though it really shouldn’t be, it’s kind of a hot-button topic in the running world. There seems to be a lot of debate surrounding it and today I want to set the record straight.
What’s the topic? The accuracy of GPS versus race course distances.
Is This Course Long? The Great Debate Over GPS Accuracy and Race Course Distances
After running a hard half marathon, or a 5K that seemed just too darn easy, you may be looking at your GPS watch, scratching your head and saying to yourself: wow, that course was really _____ (long or short).
While sometimes yes, courses can be long or short, I’m here to let you know that most often the course is indeed the correct length. If you’re receiving inaccurate data on your watch, either your GPS watch isn’t as accurate as you think, or you’re not running the shortest possible route (possibly both).
How Does GPS Work?
A quick search on the internet reveals that “GPS is made up of satellites, ground stations, and receivers. Once the receiver calculates its distance from four or more satellites, it knows exactly where you are.”
That’s all fine and good, but that doesn’t give us the whole picture. To get deeper into the topic, I looked at data from NASA, a few physics websites, and lastly, I landed on none other than Garmin.com. While GPS is very accurate, Garmin pointed out that GPS receivers are “typically accurate to within 10 meters.”
True, a GPS watch is more accurate than the app on your phone. It’s a wonderful tool for training, checking your pace, and nailing down your distance. Is it more accurate than being measured, remeasured, and remeasured again by the race company? Not usually, no. Especially with the caveat of “typically accurate to within 10 meters.”
Your location relative to the satellites in the sky matters, as does the amount of people trying to snag a signal from them. Think of it like this: when you’re lining up for a race and you press the “satellite” button on your watch, do you get a signal right away? I’m guessing depending on the firmware of your watch model, your answer may be no. Let’s also consider the “auto-pause” feature on many GPS watches. How many times have you stopped to cross a road only to have your watch pause and then once you start running again, it takes several seconds for it to restart? This happens to me on almost every run.
While GPS technology is highly accurate, it’s not the end-all, be-all — especially when it comes to race distances. There’s still room for error, even if it is small, like 10 meters.
Let’s look at more data:
I’m borrowing this example from my coach using my own data. If you run 400m on the track, you know exactly how far 400m is. It’s simple. It’s one time around the track. Start at the numbered line, end at the same numbered line. Take the same 400m distance around the track but view it in your GPS log and you get an entirely different picture.
In this picture, you see my track workout as depicted by my GPS watch. I was in the same lane (lane 1) through the entire workout. So why does my route look like I was on the football field and not on the track itself? Great question. Let’s examine.
Curves matter — a lot. Lane 1 of a 400m track is 400 meters. Lane 8 is, by all intents and purposes, just about 46 meters longer if starting and ending from the same position as lane 1. (This distance discrepancy is why there’s a staggered start for track meets.) But the straights are straight — they are 100 meters long regardless of which lane you’re in. It’s the curves that add distance. Following that logic, let’s move on to road races.
If you’re running any type of course except for a straight out-and-back, or a straight point-to-point, you’re likely to have twists, turns, and curves. Some races minimize hairpin turns, while others are notorious for them. Given the 10 meter +/- in GPS accuracy as discussed above, along with a potentially curvy route, it’s easy to see that GPS technology in these situations may not be as accurate as we previously thought.
I’m hoping by now you can extrapolate from our discussion so far the point I’m trying to drive home. If you haven’t yet, let’s continue. Keep the “curves matter” idea fresh in your mind…
To be a Boston-qualifying course, the race course must be USATF or AIMS certified. In the U.S., a course must be certified in order for a runner to be eligible for a record or to be nationally ranked.
A posted statement by the USATF on GPS use by runners acknowledges the variance of quality among GPS devices, and also mentions their effectiveness against such “obstacles” such as tall buildings, heavy tree covering, and other reception issues.
You can read the statement in its entirety here.
Running the Tangents
Have you heard of running the tangents? In geometry, a tangent is a straight line that touches a curve but doesn’t intersect it.
In running, “running the tangents” refers to running the shortest distance possible in a race. Not coincidentally, the USATF certifies courses based on a term called “SPR.” SPR stands for Shortest Possible Route. Or… running the tangents. Here’s an illustration:
If a runner is on a somewhat curvy course with lots of turns onto different streets, or perhaps just a windy road, the likelihood of a runner running the tangents perfectly, or the SPR, is very small. In a large race, it’s next to impossible to ensure you run the tangents 100% of the time.
Not efficiently running the tangents will yield longer-than-advertised race distances. So while the race may in fact be a USATF certified race course and your GPS is spot-on and your firmware up to date, if you don’t run the Shortest Possible Route, you’ll likely run farther than the distance you expect to run.
So! Let’s recap:
- GPS, while highly accurate, is not 100% accurate.
- Curves matter.
- Unless you run the SPR (shortest possible route) of a certified race, your distances will likely be long.
In Conclusion: GPS Accuracy and Race Course Distances
There are certainly races that are longer or shorter than the distance you expect to run. But be aware that this is most often not the issue. Also be aware that GPS does have its faults. If you find yourself running “long” in races, it might be worth some time studying the race course and learning to run the tangents. Better yet, if you can, run the race route ahead of time! This will help soothe any pre-race anxiety as well as allow you to really know where the shortest possible route is come race day.
Unless a race recognizes their course is long or short, please do your best to keep your frustrations inside when comparing your GPS data to the advertised race distance. Race directors put a lot of hard work into ensuring their courses are up to standard, the race goes smoothly, and runners have a good time. Let’s cut them some slack.
I hope this post helps clear up any questions surrounding GPS use, GPS accuracy, long/short race courses, certified courses, and running the tangents. Questions? Post ’em below!
TALK TO ME!
When racing, do you consciously try to run the tangents?
Does your GPS watch have the auto-pause feature? Do you remember to turn it off during a race? <—- I don’t! I always forget. Oops.