How Accurately Are Running Apps Tracking Your Distance?
Most of the time, I trust my running application. If I go the three-engine distance and get home with only 2.8 on the screen, I will be jogging around the block to finish the job. But sometimes even a smartphone can be stupid: for example, when MapMyRun captured a data point on the other side of the river that my friend was running along.
That two-second run of imagination ended up adding some serious distance to the app’s assessment of her running. Such big mistakes are rare, but mistakes come in all shapes and sizes. And if you rely on an app to track your weekly mileage or to determine your speed while running or racing, accuracy becomes very important.
How does your phone (or watch) know where you’ve gone?
Whether you have a dedicated watch like aGarmin watch or use an app like Runkeeper or Endomondo , the primary source of location data is usually a collection of satellites known as a global positioning system or GPS.
These satellites broadcast their location along with time. If your device can receive signals from four satellites, it can usetrilateration to calculate your position in space relative to those satellites.
However, inconvenient things like the Earth’s atmosphere can change the speed of the signal and introduce some error . But the more satellite signals your device can find, the more data it has to determine your location. With seven or more, the device can determine your location with an accuracy of 10 meters.
If you have an older GPS device, you may remember that when the device was new (or after having dug it out of the closet every spring), you had to stand forever and wait for a location lock. The device downloaded frames from the almanac , a list of the orbit and the status of the satellites. Newer and faster devices skip the almanac and use their own computing power to locate satellites.
This is the basic idea behind GPS. There are other constellations of satellites that perform similar functions, for example, the Russian GLONASS , and they work in about the same way. A Garmin spokesperson told us that some of their devices combine GPS, GLONASS and WAAS, a global expansion system of additional satellites and ground stations.
Phones can use these systems, plus a few more, including Wi-Fi location data (if your device can see a specific Starbucks hotspot, it knows you’re near that Starbucks) and an auxiliary GPS that uses cell towers to track data. GPS so that devices don’t have to wait so long or search so hard for signals.
How accurate is GPS actually?
With all of these systems, your phone or watch can know where you are to within 5-10 meters. Every few seconds it will mark your location and can calculate how far you have gone by connecting the dots.
Since the location is never completely accurate, your GPS signal can be thought of as a dog mostly running around you. He is your friend, so he is always there; if you know how far the dog has traveled, this is about the same distance as you.
This is where sources of inaccuracy begin to creep in: if your dog zigzags back and forth, he runs more total distance than you do. But whenever you turn at an intersection, your dog can cut the corner and close that distance. In a study published in PLOS ONE , GPS devices underestimated the distance athletes ran back and forth and overestimated how far they ran a 200-meter loop – in each case, the error varied but was in the 2-3% range. Another real-life test,published in the Journal of Strength & Conditioning Research , found that the error was usually less than 6.2% – good for their purpose (measuring total training mileage), but not something you can always rely on to tell the difference. between nine and 10 minutes mile.
If you are using a device to measure your pace while racing, there is one more thing you need to know: you can get a reading that is greater than the race distance due to the way the race track is measured. Race officials measure the shortest distance possible without cheating , which means sticking to the inside of the curves and not dodging other runners – then they add an extra 0.1% just in case. So, even if your GPS navigator is working perfectly on this day, it’s okay if it takes a long time to read.
Advances in both software and hardware have made devices more accurate over time. Chris Drouin, Data Analyst at Runkeeper, explains some of the reasons new phones can give more accurate results and why accuracy can vary from phone to phone:
- A newer phone [system-on-chip] may be better designed to reduce potential sources of GPS interference.
- Newer phones often have bigger / newer / better sensors (including ever more advanced accelerometers, gyroscopes, magnetometers, pedometers, etc. that can be used as part of a sensor fusion solution).
- New phones tend to support new phone operating systems, and both Google and Apple have made steady progress over the years in terms of their geolocation APIs.
However, the differences go both ways. New phones and operating systems are also becoming more aggressive with regard to battery use. Overall this is a good thing – we all like more battery! – but it could potentially wipe the end of the tracked activity if the phone goes into power saving mode on the last lap.
Apps can also apply their own logic to the data points they receive from equipment, for example, by removing readings that are far from the path a runner appears to be following, or by turning a sudden change in direction into a smooth curve.
How to get the most accurate reading
There are two places where your GPS will not work: among buildings and among trees:
- Buildings (especially tall skyscrapers) can interfere with satellite readouts because the signal is reflected off the building. Your device assumes the signal is in a straight line, but the bounce has added some distance that changes your mind. Buildings can also physically block the signal; GPS works best when the device has a “good view” of the sky.
- The canopy has the same two problems: it can block satellite signals from reaching your device, and the moisture in the leaves can, in a sense, provide a surface to reflect the signals. How hard you bounce depends on how dense you are in the forest, the type of trees, and the time of year.
Garmin representative told me that the clouds and storms do not have a noticeable effect on the GPS accuracy. However, if you’re running in an area with a lot of flat open areas, your device’s accuracy should be pretty good.
This makes farmland a perfect spot for accuracy, but if you’re in a remote area without a lot of cell towers, it can make it difficult to get an initial GPS fix – and cause a big jump when the device finally determines where you are, says Duane. Taking it:
If you want to record the most accurate mileage possible using your phone or other GPS device, I would recommend finding a long, relatively straight running course, free from high obstacles … especially anything solid and directly overhead [such as a bridge or tunnel] …
For added accuracy, especially if you run indoors occasionally, many devices can work with a footrest, such as from Garmin or Nike, that works with their iPhone app. You will calibrate it so the device knows what your stride is and it will take that into account in its calculations. Instead,accelerometer- based Garmin devices have a custom stride length feature, which is the same idea, but without a footrest.
So if you’re looking for the best readings for your GPS watch or phone, you can get very close, but you still have to come to terms with the fact that you can never expect your device to be completely accurate all the time. … If you are timing short distances (say how fast you can run a quarter mile or even a full mile), find a track or measured path and learn to read the markings . If you’re trying to get to a certain pace for training purposes, GPS devices can help , but you’ll want to have a backup plan (even if it’s “feel-driven”) in case conditions aren’t ideal for accuracy. However, for measuring long or light runs, or for determining how many miles you will travel in a week, your phone or watch will do.
Image by Nemanja Kozovich ( Shutterstock ).
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