So I take it there are significant numbers of monitors out there which are not using GPS speed data?  I'm talking about the subset which is logging data....this is all irrelevent if applied to data which is just going up in smoke anyhow.  Who cares if you are 1% or even 10% off if you don't even record it.

  I know our old RDS CeresII used wheel rpm to compute speed but I thought everything modern was using GPS.

Don, the issue of flat groundspeed vs terrain groundspeed is interesting. I'm assuming we farm flat acres since they are determined by aerial photography for most purposes, so I would think that flat groundspeed and area is the 'correct' thing to use from my viewpoint.

The flow dynamics Re impact sensors is a real issue, no question about that.