Hi Geert,

there are many ways to "remove spikes". That's why some report 20kts and other 17kts top speed for their boat. Actually a gps provides only a position signal not a speed signal and the devices software has to deduce the speed out of it, which requires to steps:
>translating position signals in WGS84 to distances. This is more tricky since the earth is not sphere (as asumed in many devices) nor an ellipsoid. But the error here is small.
>differentiating the distance over time. Sounds simple, but because the position signal is noisy, the error is amplified by the differentiating.
Finally you have to find an agreement what means best 100m?
Sailed track length (maybe with curves) or the fastest 100m straight distance.
At the end of the day you have measured ground speed, your preceived speed is through the water speed. A current can easily add 2kts to your measurement.

Cheers,

Klaus