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Why your GPS-enabled smartwatch may show different data than your friend’s

You’re laced up and ready to go but your GPS watch has you impatiently jogging in place while it searches for a signal. Or, perhaps you’re celebrating after a race and notice your buddies’ watches displaying slightly different distances than your own. No matter the details, at some point you’ve probably looked at the fitness device strapped to your wrist and wondered, “What the hell is going on in there, anyway?”

More: The 5 best fitness bands for backpacking go where smartwatches fear to tread

We’re right there with you. To get to the bottom of these mysteries, we asked top experts in the fitness watch industry to explain the quirks and complexities of these wearables so you — the user — has the best experience out in the field.

What is GPS, and how does it work?

Let’s start with the basics. Your fitness watch uses GPS — or Global Positioning System — to determine your exact location through a process called triangulation.

“Triangulation is a way to determine the difference between the time that your running watch receives a GPS signal and the time that GPS signal was sent to your running watch. The difference between sending and receiving a GPS signal determines how far away the satellite is,” explained Dan Lund, Product Marketing Manager of TomTom, to Digital Trends.

That process gets repeated with satellites that sit further away from the watch receiver, which pins down its exact location and continues to track the receiver throughout the activity.

“By integrating GPS technology in a fitness or sports watch, the watch measures a series of points that are used to accurately derive metrics like distance, speed, and pace. These metrics give runners real-time insight into their running performance,” Lund added.

What about elevation?

Elevation information is available via GPS just like horizontal position, but Lund and Garmin Product Manager Joe Heikes says the data is typically very noisy, and usually calculated through a website or app using a digital elevation model.

For more accurate elevation readings, many fitness watches use barometric altimeters to measure the pressure changes at different elevation levels during your workout, providing real-time data on elevation change. “This is 3D distance vs. 2D distance,” Lund explained to Digital Trends.

“We’re putting a barometer in more and more of our wearable products because it enables the ‘floors climbed’ feature,” Heikes said. “However, I think customers should understand that, for the vast majority of runners, there is very little benefit to this measurement.”

How much distance is lost if elevation isn’t taken into account?

Evidently, not much. Heikes provided some numbers to illustrate his point.

“A quick mathematical check shows that a 10-percent grade over one mile creates only an additional eight meters of distance as compared to one mile over a flat. While a 10-percent grade may not sound like much, that would be because one has not actually tried to run a mile up a 10-percent grade!”

The bottom line is that, in most cases, the amount of distance lost using a 2D instead of 3D measurement is likely a small fraction of a percent.

“In most [Garmin] products, we do not take elevation changes into account in the distance calculations. Except in rare cases, it really doesn’t matter,” Heikes pointed out.

Why does my watch show a different distance than my buddies’ even though we ran the same route?

This could be due to a number of reasons. The main factor is probably that you all wear different devices which use different ways of receiving, logging, and filtering the GPS signal.

You might also notice larger discrepancies over longer distances. This is due to slight differences in GPS errors that have accumulated over distance.

“The battery-saving option available in some devices may decrease the accuracy of the GPS measurement,” said Suunto Product Manager and triathlete, Markus Kemetter to Digital Trends. “Nowadays, the GPS technology has developed to be very good, and distance variation between devices is often below one percent.”

Different algorithms applied to the data also create minor discrepancies. “Running applications smooth out the data to increase the accuracy of GPS,” explained Lund. “That can have an effect on the distance displayed.”

Obviously, there’s tremendous potential for human-sourced differences, too. Any extra steps you take — whether to an aid station or a pit stop in the bushes — count directly on your watch. That is, unless you pause it.

By pausing your watch, you no longer receive GPS signals, so the only way to make up any distance lost when you forget to un-pause is to manually edit the activity in the app.

“We feel very confident in the running distances calculated by our Garmin Forerunners, and in my experience our distance error is usually less than about 0.7 percent, which is about 11 meters per mile,” Heikes said, and continued to explain that 11 meters leads to a three second difference between the time your watch beeps and someone else’s beeps at the one-mile marker.

How strong is a GPS signal?

“For users on land or sea, a GPS signal is about as strong as a light bulb would be to your eye when it’s a few miles away,” said Spirent Head of Marketing Solutions & Services, Simon Loe to Digital Trends. Furthermore, Spirent is a company who makes equipment designed to simulate conditions capable of potentially interfering with GPS devices.

Of 32 satellites in the GPS system, Loe said a receiver must be able to “see” at least four in the sky in order to function properly. “However, seeing more satellites makes it easier to establish a more accurate position,” Loe said.

Because the signal is weak, it’s influenced by a range of factors from natural effects, to interference from other signals being transmitted in the area.

What types of things cause interference?

The biggest threat to a GPS signal is, quite simply, “stuff in the way.” This includes tall buildings in urban areas and dense tree cover in the wilderness.

“In deciduous forests, as leaves fall in winter, a watch may record a different path from the same route taken in summer,” Loe said.

Obstacles like heavy foliage and skyscrapers also reduce or completely block GPS satellite signals. “They also create other problems such as when the signal bounces off a building before it reaches the watch, in which case the GPS receiver in the watch will think it is farther from the satellites than it really is,” Heikes added.

This phenomenon is called multi-path, and even though there exist algorithms which try to deal with multi-path interference, it still creates errors.

Experts agree the best method for avoiding interference is to run in a clear, wide-open area. However, they understand this isn’t an option for many, so some fitness watches come equipped with an additional feature called GLONASS, a Russian system of global positioning similar to American GPS.

The GLONASS feature allows the watch to see the additional Russian satellites, thereby reducing the potential effect of blocked GPS satellites. Users should keep in mind this feature comes at a cost to the watch’s battery life, since it has to send a second signal, and provides no benefit to those with a clear view of the sky.

Why does my watch sometimes take longer to locate a signal than others?

If you’re using your watch for the first time in a new area, it may take longer to locate you than it would in a spot where you’ve already tracked an activity.

“The speed of getting your first fix is dependent on how well the device knows the current satellite constellation around you,” said Kemetter. Most devices remember the last location used and therefore find you faster upon turning on.

“Many devices also use satellite orbit predictions,” Kemmeter added, referring to global predictions of where satellites orbit at any given time. “This prediction data is valid for a short time and can be refreshed using a data connection to the device.”

That means frequently syncing your watch to its corresponding app on your phone — e.g., Garmin Connect or SuuntoLink — helps it behave the way you want it to.

Can’t I just use my phone?

Yes, absolutely. But it’s going to drain your battery rather quickly, and you likely want to save some juice for emergencies or ordering a recovery burrito.

Companies specifically design GPS watches to tracking your outdoor activities, which means they typically have smaller, lighter, more durable, and longer lasting batteries.

“In mobile phones, the focus of development isn’t usually on optimizing the antennae solution and the GPS software to perform well in outdoor sports,” Kemetter pointed out.

Companies such as Suunto, TomTom, and Garmin prioritize distance and pace measurements in their fitness watches to provide a higher level of accuracy than what might be available using an app on a smartphone.

“Our products are purpose-built for athletic applications, unlike a cell phone,” Heikes said. “That means that even the GPS implementation is fine-tuned for those applications. We don’t simply take the raw data coming from the GPS receiver, but we add our own smarts when we calculate distance and pace or speed.”