I know perfectly well what you are saying and you are correct. The thing is I boat on an inland lake with no current so my GPS agrees with my radar gun and my that is my speed in reference to the water I am running on. You are correct that they can use GPS to determine kilo speeds, but they use the average of two passes in opposite directions. They do this because they, and all boaters want to know how fast the boats are in reference to the waters surface and pitot tube speedos vary to much from gauge to gauge to be fair. If there is a current or wind in the kilo runs the speed could be faster in one direction than the other and the average of the two will be the actual boat speed in reference to the water. I guess my point is this. Boats run on water and the important speed is the one in ref. to the water they are running on. As for the math you and Audicity keep preaching, boat speed (in ref. to the water) equals GPS speed North plus GPS speed South divided by two. If you have another formula please share it with me, and provide details. I am a mechanical engineer and i like math. By the way I am pretty sure a few OSOers are using their GPS max downstream in a fast river as their boats speed in ref. to the water. But only a few.