In essence, I want to end up with a section viewport that shows something like this:     The green represents the 'ground', which of course the bottom of any wall and its foundation is going to be buried in.   In actual fact what I'm really interested in is the solid line that represents the surface of the ground: I want this to be generated from what I model in 3d. So, I'd also settle for something like this:     I've been trying to do this us