You&#x27;re probably right, it&#x27;s probably a low sampling rate. (Either dude was going fast or the app is really casual about taking measurements...!)<p>Also a possibility: the phone is picking up a gps &#x27;echo&#x27; reflecting off a wall.  We like to think of GPS as this magical, perfectly accurate reporting of our cartesian coordinates, but in practice it&#x27;s a lot messier.  There are places that <i>consistently</i> 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).<p>The wavelength used for GPS is pretty well absorbed by water, so you have situations like this more often than you might think.