Exactly. You can find several websites of amatuer that did the swing test. And from memory I think is swung only a few mtr within a few hours. So any Track readout achieved in minutes will be accurate within 10 centimeters. I don't know of any other more conventional method that allows you to measure out a a distance more accurately on water.

But the real trick is to have the GPS log a track and simply look for a 500 mtr stretch of sailing where the average speed was highest. In fact you don't even need a start and end bouy. Just an body of open water, good water and a crew trying for 10 minutes to get the highest speed going for at least 50 seconds.

Later on the PC you can walk through the recorded track and find the track segment with the highest average. As all points on such a small geographical area will all have the same build-in off set it means that the RELATIVE distance between each point on the tack is very accurate and so to the averaged speed. I think we can trust the internal clocks to be very accurate in GPS units.

As a matter of fact the build in error could be 100 km and the results would still be very accurate; just as long as the error changes only very slowly per minute. And this is the case with civilian GPS. The error changes relatively slowly. So any measurement over a short time span will be much more accurate than one would think at first. Just don't intepretate this to mean that MOMENTARY GPS speed is accurate as well as it simply isn't, but it will be to much detail to go into that,

Wouter