Oh, and BTW, it's not mentioned, but it would be quite interesting, and much more of a burn, if Elon is actually making some of the charges, like driving in loops, with a proof via GPS logs of the test drive.
This is also used in surverying to do something called differential GPS with two receivers: one GPS receiver fixed in a known location, and another to measure GPS locations relative to the first GPS in the known location - absolute error is completely removed by this process (http://en.wikipedia.org/wiki/Differential_GPS claims 10cm accuracy). If you think about it, two receivers separated by a little space is more or less equivalent to two receivers separated by a little time, so you'd expect a similar relative accuracy of 10cm.
> GPS is only accurate to a certain amount (maybe 1.5 metres, maybe 10 metres)
Statistical civilian GPS accuracy is better understood than this. It's 7.8 meters (25.6 feet) for two standard deviations (i.e. 95% confidence):
I gave up looking for an accuracy figure on wikipedia, but knew I had seen 1.5 metres in the past, and that 10 metres is too inaccurate. Thanks for the link.
It all has to do with the statistical error bound. For a specified accuracy, one must also state the deviation for which that error is true. My point is that GPS doesn't have a specific accuracy, always true, with a probability cliff on each side. For a given accuracy specification, one must always include the probability for that accuracy.
Regardless, I don't think that's a huge sticking point to this story. Broder was at the charging station - it's possible he wanted to see just how long it would take until the battery shut down.