Another great resource I have found for topography mapping is LIDAR data. Kansas offers free access to the states LIDAR catalog http://www.kansasgis.org/catalog/index.cfm. This data is extremely accurate. I use a free program called QGIS to map out a variety of useful layers (slope, aspect, terrace profiles ).

Personally, I believe the companies running precision ag are in for a bumpy ride in the next few years. I watched a video a few weeks back of a farmer in canada who used open source software to make his grain cart fully autonomous. https://www.youtube.com/watch?v=Ybxhvlyw-X0.  That really inspired me to try and build this project. If someone like myself with no background in coding/electronics can figure this stuff out, Imagine what a few kids with a background in computer science could do. $20,000 for a full autosteer system, pfft.