-- Last Updated: Jan-30-13 4:03 PM EST --
Handheld GPS can't account for everything
- there are caveats and exclusions.
A receiver compares the bit sequence received from the satellite
with an internally generated version.
By comparing the rising and trailing edges of the bit transitions,
modern electronics can measure signal offset to within
about one percent of a bit pulse width.
Since GPS signals propagate at the speed of light,
this represents an error of about 1 to 3 meters.
I doubt few people could ""accurately"" measure
skeg_up vs skeg_down over a set distance and
get repeatable results.
A measured course on water is tough to set-up.
Bouys move and each gps will measure just
a bit differently from another one.
Tooo many variables in human, nature, equipment combo