Thanks, D.
What an awesome website! I love it!

But a little knowledge is a dangerous thing. Now I learned that a GPS is not as accurate as a DGPS.

I realize my original question is difficult to answer, because if you are on a boat, with no other way to verify your speed, how can you know whether the GPS is accurate or not. But I was hoping maybe somebody had gotten some independent verification of the validity of their GPS readings. D. was able to verify it against his car speedometer, but has anybody had corroborating verification when on a boat?