You say you want non-flat area.....I have to ask if that is really what you farm? 

If a 'square' field has a hill in one side, are the ends crooked and non-parallel?  If you are truely farming  non-flat acres they would have to be.  I doubt they are.  I'm betting your world is layed out from a celestial reference point just like everyone elses, so assuming a field with a big hill in one side, I would bet the rows on that side, measured on the ground, are longer then on the other side and the ends are in fact parallel.

I'm almost sure all of our Agleader monitors use GPS speed.  I know they don't go all wacky when the tracks are installed like the dash does, and anything related to the driveline would.