This is an interesting discussion and all, but I'm wondering if anyone has actually calculated the difference between the flat area of a given field and the terrain area.

I've got this strange feeling this is one of those things which are discussed by thinking farmers without enough to fill their time.   I'm picking a number out of the air, but I'll  bet that it's hard to find a field where the difference amounts to more then say 2%.  Ted, since you brought this up, have you looked at actual fields and calculated how much of an issue this really is?