How Accurate Is Your Running Watch Really?
If you’ve been in the unfortunate position of missing out on a PB because a race course measured longer on your GPS running watch than advertised, then join the club. After chasing a frustratingly elusive 10K PB for years, it recently came within my reach. I was in good form, training had gone well, and I felt strong and ready to race. The race also went exactly according to plan: I ran strong, kept a steady pace and had a good finishing kick. You can imagine my frustration, then, when I discovered, post-race, that I nailed that PB when my watch struck ten but didn’t clock an “official” PB since the course measured 200 m too long. Ugh.
Which got me thinking. Why is there often a discrepancy between manually measured race routes and GPS measurements? Does this phenomenon boil down purely to the reliability (or lack thereof) of GPS running devices? Or do other factors also come into play? Here’s what a few technology experts have to say.
How race route distances are manually measured
First off, let’s have a look at how USATF certified courses are measured. In terms of the USATF Course Measurement and Certification Procedures, a Jones Counter attached to a bike is used to physically measure “the shortest possible route that a runner could take and not be disqualified”. This involves coming “within 30 cm (one foot) of all corners”, going “straight through S-turns”, and moving “diagonally between corners when crossing a street”.
But before these measurements are taken, an intricate procedure involving at least four calibration rides on a straight portion of a paved road must be completed. This is followed by at least two rounds of full course measurement by a certified member of the USATF, recalibration of the bike and then determining the constant for the day. Only once this constant is known can the proper measured course length then be confirmed. Note that a “short course prevention factor” of 0.1% is also built into the course measurement and calculation process in order to ensure that the course does not measure short.
How GPS distance measurements are taken
Next, let’s look at how running watch GPS measurements are taken. While most runners know that GPS devices measure distance and speed with the aid of satellite signals, few know the details of how this actually works.
Contrary to popular belief, orbiting satellites are not in constant communication with your running watch once a signal is obtained. Instead, it checks in with your watch at short intervals. According to Peter Ranacher from the Department of Geoinformatics of the University of Salzburg in Austria, “when you take your GPS for a run, you collect a so-called GPS track. A GPS track consists of a sequence of consecutive GPS positions.” From there, your device will then do the work to calculate the distance from each check-in point and your distance traveled and speed from there.
Potential reasons why GPS-measured distances vary from USATF-measured ones
When comparing the USATF and GPS measurement methods, it’s not hard to explain why they often produce slightly different results. Possible explanations include:
- Not running the shortest possible route. Since USATF measurements focus on following the shortest possible route between two points, i.e. turning the corners as tightly as possible, it’s only logical that you’ll run slightly further on a very congested race course.
- Logging extra distance to get to aid stations, medical tents or portaloos. These structures are often placed slightly off-course in order to avoid congestion, thereby causing you to add a few extra steps of running to get to them.
- Dodging and weaving around other race participants. If a race course is really congested, faster runners often dodge and weave around slower ones, thereby clocking extra mileage that adds up in the end.
Additionally, in a study published in the International Journal of Geographical Information Science in 2015, Ranacher and his team discovered the way in which GPS devices calculate distance could lead to two potential errors. First off, if satellite check-ins with your device are too infrequent, it may underestimate your distance traveled. “Let’s say you run along a circular track, but your GPS only records four positions during that time. Your GPS track looks like a square, whereas it should be a circle,” Ranacher explains. Infrequent satellite check-ins can be caused by factors like an inferior or dated GPS device, but also by environmental factors, like an overcast day or the presence of a closed tree canopy or numerous high skyscrapers.
The potential second type of error is referred to as a measurement error. This basically boils down to very slight location errors each time the satellites check in with your device. When your device then measures the distance among these slightly off points, the line is a bit longer than the actual distance you ran. Which, of course, adds up if you cover a long distance.
Not all running watches are created equal
It’s important to note that not all GPS running watches were created equal. Ranacher and his team deliberately used low-quality GPS watches to make sure that the anticipated errors would be visible. But what about top-of-the-range devices, you ask? Would they also be susceptible to these errors? Yes, but to a much smaller extent. “I’d reckon that the error for better devices is much less, but I’d also reckon that it is still there,” Ranacher speculates.
Is it still worth investing in a GPS running watch?
So, taking all of this into account, is it still worth investing in a GPS running watch? Ranacher sure thinks so. “There is one more thing I’d like to say: GPS is a fantastic system. The statistics are just not in favor of GPS when it’s being used to collect distances,” he says.
So before you run your next race, be aware of the potential limitations of your GPS watch and plan your race accordingly. Also, keep in mind that the way in which you run your race could have quite an impact on your final distance measurement. Always aim to run the shortest possible line!
And as for that frustratingly evasive 10K PB, I’ve made peace with the fact that it’s not mine to claim. Yet!
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