From an interview in Fortune with John Deere’s CEO:

How precise is agriculture getting?

On our equipment — large combines, large tractors, large sprayers — we have proprietary GPS that enables what we call auto-steer. We don’t even want the operator to touch the steering wheel. Going up and down a field a mile each way, if the farmer has a 60-foot boom on the back, 30 feet on each side, there’ll be only a two-inch overlap. Prior to this, a farmer would have overlapped by at least six feet.