I am looking at the 76S. 

I want to be able to measure odd shape fields, like a pivot corner. How accurate will it be?  Like a 5 acre pivot corner will be + or - how much in acres?  or  Plus or minus what percentage? 

And second, I was thinking of putting it in the combine to measure the distance for yield checks.  I have scales on the grain cart.  The cart tractor always has to drive the same distance as the combine to be able to get the area for yield check because it also figures the distance, 4755.   I want to measure the distance by myself in the combine and then just ask the cart driver the weight.  What would be my percentage of error in distance measurement?  

I would be using an external antenna. 

Thanks.