You're probably right, it's probably a low sampling rate. (Either dude was going fast or the app is really casual about taking measurements...!) Also a possibi

You're probably right, it's probably a low sampling rate. (Either dude was going fast or the app is really casual about taking measurements...!) Also a possibility: the phone is picking up a gps 'echo' reflecting off a wall. We like to think of GPS as this magical, perfectly accurate reporting of our cartesian coordinates, but in practice it's a lot messier. There are places that consistently report an inaccurate location, because some non-direct path length transmits more strongly than the direct line to the sky (which may be occluded by trees, for example). The wavelength used for GPS is pretty well absorbed by water, so you have situations like this more often than you might think.