This is the John Deere autotrac system working on our 6930 drilling spring barley last week. The tractor is GPS guided pulling the drill at 4m intervals across the field (we hope) The system was added to the tractor last summer to help reduce the costs of further chemical applications by not overlapping the passes and thus the fertiliser or pesticides subsequently applied. Once set up the system is fairly accurate. This should then make sure that the tramlines for the sprayer are exactly 36meters apart. A small inaccuracy of say 10cm, over 9 passes (9x4=36) equates to the tramlines being 90 out in either direction, which is quite a lot.