Mark, I'm no expert on this (although I bet some of the ex-pilots on here are), but there are many many variables to GPS, the handset yes, but also which satelites are being picked up and whether there is a variable offset. When I was still serving we were issued mil codes for GPS as the US (who own the system) would insert a variable offset in the system to ensure it could not be used for military purposes. That said, I beleieve the GPS would still be accurate relative to itself, i.e. if you used the same GPS to mark out the whole course, the distance and angles between marks would still be right. This might have changed though as I heard the offsets were reduced or removed. When I did some time on a survey boat, they would set up ground stations at known locations for coastal work so they always had a absolute reference (known as Differential GPS I think).

Apparently the new Euro system we are all being taxed for will be very accurate and not use any offsets.

Any experts out there?