never thought of it like that before, but I suppose that waves have an "uphill" bit above the average flat sea level, and also a "downhill" bit below the average flat level.

Second thoughts. If you get a piece of string a foot long and put lots of little waves in it, it doesn't measure 12 inches until you pull it straight again.

Maybe we should have a modification to the GPS which gives Distance Including Up and Down, as well.