Allan, the lightbar I have is "ancient" but it will calculate acreages of any shaped field by just driving the outside edge.
I have run a neighbors planter tractor wich is equiped with newer gps and his planter has individual row shut-offs. There you have to establish a "boundary" ( end row, or whatever folks in NE call them) for the gps to know when to shut off seed flow. We are using WAAS satellites (bottom of the barrel stuff) and can only establish these boundarys right before planting the field due to sattelite drift. There is newer stuff out there (RTK etc.) that is much more accurate, but needs a subcription to use it $$$$$.
So in trying to answer your question, I am thinking that even the RTK is not capable of making a field boundary this year and repeating that exact same path next year. Probably would be close though.
Others will know much more about GPS than I do, much more for me to learn.