GPS's are not that accurate, you can't use it to measure top speed of humans. Anyone know how much off GPS's can be ? I guess in 10 m/s they can be +- 1 m/s off

GPS's are not that accurate, you can't use it to measure top speed of humans. Anyone know how much off GPS's can be ? I guess in 10 m/s they can be +- 1 m/s off for good quality and all down to +- 50% for a bad GPS. Anyone know how exact accelerometers are ?