Its like air speed vs ground speed, just substitute water for air. For towing, over-land speed isn't important, its over water speed. If you board at 20mph on a lake with GPS and you're set up the same on a 2 mph river, then you're really boarding at 22 mph (water speed -assuming traveling up current). Then, when you turn and travel down current, you're only going 18 mph (water speed). So, your GPS based PP is roughly off the mark plus and minus the current's speed.
Never Mind The Bollocks, Here's The X-Star
Last edited by deminimis; 06-29-2011 at 04:34 PM.