Masthead altitude
Steve, your wondering about Lat/Lon or perhaps Attitude made me think of another area of worries: we all know that the density of fresh water differs than sea water. And that will make a boat ride higher or lower on the water. Consequently, while the distance from the water level to the top of the mast is normally given as part of the specs, I would imagine that to trust that measurement, one would have to also know the density of the water where the boat was first measured, and also where the boat is currently located, thus being able to calibrated the measure accurately in order to get the precise altitude of the masthead. If you have had the fortitude to read this thread until now, I won't have the turpitude of carrying any further and will go get a beer, hopefully not in solitude.