Sunfish, OZ,
I think you need to update with what Australian agriculture is doing, RIGHT NOW, perhaps pay a visit to the next AgQuip at Gunnedah.
Accurate (much better than most aviation applications) GPS positioning is already being used, has been for years. All the comments about SBAS/WAAS and transport/agriculture was true 15 or more years ago, but things have moved on.
Remember we had differential GPS available for surveying when?? Twenty five plus years ago, with a modern equivalent now. Have a look at the navigational accuracy that is available now, for safe transit of shipping through the Reef, Torres Straight etc.
Tootle pip!!