Why can we average together data from the GPS units in our mobile phones and laptops to measure plate motions?
The main problem with crowdsourcing GPS is that it works really badly indoors because of the poor (sometimes non-existent) sky view, which is where most people spend most of their time. But even outdoors, the 30m accuracy provided by most mobile phones and laptops means that we would need prohibitively high numbers of data points* to get mm level accuracy, which is what we need for measuring plate rates. (For geodetic grade GPS stations, we use the signal from the GPS satellites in a fundamentally different way from the way handheld devices use it, in order to get mm level accuracy at each station.) Moreover, the rates we are measuring are so slow that they could not be measured over the course of a single day (one would need sub-mm accuracy for that.) So people would need to put their laptops back in the same place, within a mm, every day in order to track velocity over a longer time. Secondly, there is a geophysical problem with using widely distributed GPS signals to track plate mot
