Plus or minus a couple of feet? That's the accuracy of a GPS system, or so I've been told.
Well, you don't want irrigation water or rain to just stand around, you want it to drain off the field. It really isn't good to have water standing on your field.
Back in the day, we used to have these arms that stuck out on either side of the planter. You put the one on the side toward the unplanted section down and raised the other. It would mark where you needed to drive on the next pass. Our main worry was having a tube plug up and not planting a couple of hundred feet of row. I don't see how GPS would help that problem.
You can look at the plants. That will tell what kind of shape the plants are in (healthy or sickly) and that's a good indication of what your yield will be like out of that section. If you wait until harvest, it's too late to do anything about problems.
Or you can get your ass out of the air conditioned cab and walk the field looking at the seed pods/corn ears. Check a few every hundred yards or so and see if they are filling up. Kind of low-tech but it works and the cost is a lot less.
Jack