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Your GPS Is Lying to You About Distance

Turns out the navigation devices routinely overestimate distance traveled. Why that quirk hasn't—and won't—affect cyclists.

By: Erin Beresini -Dec 7, 2015
Outside Live Bravely

In September, researchers from Austria and the Netherlands published a study titled “Why GPS Makes Distances Bigger Than They Are,” which explored the fallibility of navigation devices. But while the paper points out some interesting facts, those facts, one expert says, are neither revelatory nor particularly useful for GPS users.

“I don’t know why they’re making such a big deal about it,” says Mohinder Grewal, a professor at California State University-Fullerton who has published several books on GPS and the algorithms used to sort its data. “It’s very well known that there are measurement errors, but they are bounded.”

Before we get into what Grewal means by “bounded,” let’s take a look at the paper. In a nutshell, it tells us that our GPS devices routinely overestimate the distances we run, bike, and hike—something you may have suspected if you ran an official 10K to see your GPS device say you ran 6.3 miles. The reason for that overestimation of distance, or OED as the researchers write, comes down to three major measurement errors.

The first is positioning error, or the fact that there’s a difference between where you actually are and where your GPS thinks you are at any given point in time.

“I don’t know why they’re making such a big deal about it,” says Mohinder Grewal. “It’s very well known that there are measurement errors, but they are bounded.”

The second error is the variance of the GPS measurement. Even if you don’t move, the samples your GPS takes won’t each be in the same location. In other words, your samples will form a cloudlike cluster of points around your actual location. The smaller that cluster and the closer it is to your actual location, the smaller the OED will be.

The third is the autocorrelation of GPS measurements. If each measurement is off from your actual location by approximately the same amount, they’re said to be highly autocorrelated, and they won’t contribute as much to OED.

A GPS device will take a sample of your position on a regular time interval—say once per second. Then it will draw a line between each sample to determine the distance between them. Because of the errors mentioned above, the researchers conclude “distances recorded with a GPS are on average bigger than the distances traveled by a moving object.” Then they create an equation that links the three errors to calculate overall OED.

There are a host of other well-known reasons GPS devices aren’t spot-on, Grewal says. The earth’s atmosphere (the ionosphere in particular) interferes with satellite signals, for instance. So can trees, tunnels, and overpasses, as anyone with satellite radio knows.

So what to do if you want the most accurate distance measurements possible? Grewal recommends making sure your GPS is WAAS enabled. WAAS stands for Wide Area Augmentation System, “a system of satellites and ground stations that provide GPS signal corrections,” Garmin explains, that’s provided through the Federal Aviation Administration. (Other countries, though not all, have or are developing similar systems with different names.)

WAAS was originally developed for better precision in maneuvering GPS-enabled planes. “That completely bound the error within a meter for landing airplanes,” Grewal says. In other words, the variance of GPS measurements in planes using WAAS should never be off by more than a meter of the plane’s actual location—the variance measurement error is therefore “bound” by one meter.

According to the equation developed in the recent study, the smaller the variance measurement error, the smaller OED will be. The error of the WAAS-enabled personal GPS devices is bound by three meters, while GPS devices without any correction are typically off by 15 meters, according to Garmin. Therefore, GPS devices with WAAS should have less OED than those that don’t.

Ultimately, Grewal is uncertain if the equation this paper presents will affect GPS at all. It was published in the International Journal of Geographical Information Science, he points out, a publication that deals more with surveying rather than the science of satellite-enabled navigation. For that, he says, researchers usually target Navigation, the journal of the Institute of Navigation.

Perhaps the equation from this paper will one day factor into how your GPS determines distance traveled, helping to eliminate OED. But for now, enjoy the bonus mileage.
Filed To: GPS Devices, Navigation, Fitness
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This article is ridiculous. At the same time she gives her own summary of the paper and concludes that your GPS watch always overestimates your distance traveled, she quotes a GPS expert who says, basically, that the paper is irrelevant and of no practical significance to owners of GPS watches. Why ask an expert if you are going to ignore what he says?
I read it to mean that the paper is irrelevant because "everyone knows GPS devices are not precise". Not that the paper was without merit, but that, to Grewal, this is not new information.

SPR has nothing to do with the point they are making in the article - that a GPS will consistently over-report the distance traveled. According to the article, if you ran the SPR, the GPS would still over-report the distance traveled.
"But while the paper points out some interesting facts, those facts, one expert says, are neither revelatory nor particularly useful for GPS users." The expert is saying we already knew what the paper proves, and it means nothing to a person who owns a GPS watch.

The paper is analyzing a theoretical situation in which there is no interpolation error (when you go around a turn the GPS takes samples at a high enough frequency that its straight-line distance calculation doesn't shortcut the actual distance), and in which the overall distance is calculated by simply summing up the straight-line segments between points without doing any filtering of the data. Neither of these is true for real GPS devices. In fact the first one is often used in articles like this to argue that GPS distance measurements will always be short!

Anyone who has tested actual GPS devices knows they sometimes slightly overestimate the distance, and sometimes slightly underestimate the distance. The idea that they always overestimate is simply untrue.

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