Hikermore,

They call it "Dead" reckoning from the old days of sail. You assume no currents or wind drift, as though the water was DEAD (ie., not moving). Of course, they began to use it knowing current and/or wind drift and correcting that in, but the idea is that the water is dead or not moving. It works even better on land where wind and currents don't move you off course.

Kevingg,

Your explainations are almost as good as your knots -- which is to say VERY good. Thanks!

Respectfully,

Jerry