Are GPS Watches Accurate?

A closer look at the distance-tracking capabilities of GPS running watches

You lace up your shoes, plug in your tunes, and of course, click on your watch; once that bad boy’s GPS connects to the satellites you’re ready to run.

Since you’ve started logging your miles with a GPS watch, you wouldn’t dare hit the road for a run without it. But, have you ever wondered just how accurate your gadget’s mile-marking capabilities are?

Many times this question first presents itself to runners after a race, when the distance recorded on their watch doesn’t match the distance of the course.

This likely occurs due to a number of different factors, including the fact that race courses are measured by the “shortest possible route” (meaning corners and turns are cut as closely as possible), so it doesn’t necessarily mean that your watch is broken or inaccurate.

“I have to say I'm always fairly impressed with how accurate GPS watches are,” says Kieran Alger, an Editor-in-Chief and freelance writer covering running, fitness, and technology and an avid runner. 

Since 2009 Alger has finished 20 marathons, 6 ultra-marathons, and 45 half-marathons, including the Marathon du Mont Blanc, Race to the Stones 100km. Last year in December he ran 31 half marathons in 31 days and his current race calendar has the Marathon Des Sables, the Boston Marathon, and the London Marathon—the equivalent of eight marathons, on three continents in 20 days—lined up for this year.

As a tech writer, and with all the running he does (which is clearly a lot), Alger has had the chance to test almost all of the major GPS running watches on the market.

“[I’ve] run set distances from town to town where there are signposted mile markings and against Google Maps,” he told me over email. “In my experience they've mostly come in within 0.25 miles of what the road signs say.”

But that’s not the only way he’s put the many watches he’s run with to the test. Alger’s faith in the accuracy of GPS watches was not won so easily.

“I've been known to run measured races wearing five watches at a time to test the accuracy,” he said. “And there's almost always a variation against the course even when you get to run the magic blue line. It's rarely more than half a mile though.”

By the “magic blue line”, he means the shortest possible route for the course—the line by which it was measured.

He’s also tested GPS watch accuracy by comparing route distances against a GPS app called Endomondo on his Android Smartphone.

“Again I've done this while wearing lots of watches on the same run,” he said. “The app tends to land somewhere in the middle of the range of distances you record on the GPS watches.”

He made a point to note that when comparing the watches to an app, his tests are usually done with a “top-of-the-range” smartphone equipped with a top-notch GPS chip. “People using lower end phones might not have the same experience,” he said.

Still, with extensive experimentation that has produced less than perfect results Alger remains “fairly impressed” with the accuracy of GPS watches.

That’s partially due to the fact that as a runner, aspects other than distance tracking are more important to his training.  

“Runners at different levels will look for different things but for me personally, when I'm trying to hit a personal record, it's vital that the real time pacing is as real time as possible,” he said.

He explained that as he was aiming for a sub three-hour marathon in London last year, it was crucial to maintain a disciplined pace.

“You can only do that if the watch responds quickly to your shifting gears, otherwise you're constantly adjusting,” he said.  

He also mentioned that a watch’s heart rate capabilities are of great importance to his training.

“Just as the five seconds per mile faster or slower can kill your race, a few beats per minute can mean the difference between blowing up at mile 17 or staying below the threshold where your legs start to fill with lactic acid,” he said.

Alger says that a watch’s battery life is also an important factor for him.

“If your GPS running watch can't last the distance it's kind of useless,” he said.

He told me that his favorite GPS running watch model changes quite often, but at the moment he’s a big fan of the Polar M400.

“It's wonderfully simple to use and I love the fact that it also has activity tracking,” he said. “It's a GPS running watch and fitness band in one and it's in the mid-range in terms of price.”

Alger also recommends the Garmin Forerunner 920XT and the Garmin Forerunner 620 for the impressive amount of dynamic stats they’re able to track.

However, for runners who want to explore the market further, he offered a few watch shopping tips.

“The first thing you need to do is ask yourself what kind of runner am I or do I want to be,” Alger said. “If you're only ever going to want to track your 5km runs around the park then you won't need a big expensive fully featured watch like the Garmin Forerunner 920XT.”

In fact, runners at this level may not even need a watch at all. Alger said that a heart rate chest strap that can pair with an app on your phone may be more than enough for most recreational runners.

However, if you’ve got lofty goals, then you’ll likely want to consider investing in a well-equipped watch.

“If you're more serious and you're going to be chasing personal bests every year then I'd advise investing in something that has good running dynamics to help you improve your form and good supporting web and app tools to go back and review your stats,” Alger said. “Community can be important too for motivation and this is increasingly a big part of the watch manufacturer’s web offering. So check those out too.”

The bottom line: runners have lots of options to choose from when it comes to GPS watches, none of them can promise perfection, but if you take the time to choose the one that will work best for your training needs, you likely won't be disappointed.