To get position from accelerometer data you have to integrate it twice. Accelerometer data is noisy, and integrating noise quickly leads to large deviations. Th

To get position from accelerometer data you have to integrate it twice. Accelerometer data is noisy, and integrating noise quickly leads to large deviations. That's why you need some kind of absolute position reference, like GPS, to not get bigger and bigger errors. Here, they seem to estimate which stations they are at or between, and use that as absolute references. Your running app is using accelerometer data to recognize when you're taking a step, and combining that with GPS data to measure how far you're running. So if it measures 100 meter using GPS and counts 100 steps during that time, your step length is 1 m. Garmin watches uses this to measure your running distance on treadmills, but they want you to calibrate it by running with GPS on a flat surface outdoor first. I don't know if any watches or apps use this in combination with GPS to make better position estimates when the GPS reception is poor, my guess is no because it's really only useful for distance, not position, and generally when running outdoors you're not running in completely straight lines. But it should be possible to estimate distance and combining it with their routing software to guess which way you took for short parts of the course.