That is very interesting stuff! The farmers are using RTK, as I'm sure you know, for sub inch auto steer accuracy, but it comes at a much higher price than $400/$800. One of the local RTK installations (Real Time Kinetics) has the base station mounted atop the local "high rise" grain elevator to cover a large portion of the valley. The good thing is that several farmers can share one base to minimize cost for auto steer and other GPS apps . I think there is a pending market for consumer grade auto steer for lawn mowers and fertilizer spreaders. (Not sure how the robo mowers work) A GPS controlled lawn fertilizer applicator with auto shut off is not far fetched, in my opinion.
I'm an old machinist who became acquainted with NC (Numerical Control) back in the late sixties in a machining environment. We used punched tape to control the milling and drilling of holes etc as there were no micro computers back then. It later morphed into CNC (Computer Numberical Control) and prices stated dropping dramatically to the point where hobbyists could afford it. CNC wood routers are common today and I have used mine extensively for producing components for the Palen Archtop Guitars shown at the link below. (Shameless plug but I actually quit building them in 2012.)
What I'm getting at is there seems to be a corollary between the CNC and GPS that keeps popping up for me as time goes on. You mentioned "Drive around the field and every few seconds log your location and elevation" and that is basically what I did to one of my hand carved archtop back and top plates to get it into electronic format. The archtop plates start out about 1" thick and end up as a carved contour about 3/16" thick. Digitizing the carved plate (spruce or maple) involved mounting an electronic probe into the spindle of the CNC router and traversing the entire surface in a grid of points about 1/16" to 1/4" apart and taking elevation readings at each. This created an electronic file that could be read into the PC creating a point cloud and generating a surface in turn. This surface could then be viewed on the screen and tweaked as desired and even a solid model could be created.
Sorry to ramble but but it seems that there is a tremendous opportunity out there for consumer grade GPS apps.
Edit: Forgot to mention that the CNC router could then be used to machine subsequent parts eliminating much of the labor intensive hand carving.