I am having problems with creating a sloped terrain that is confined to a 11' wide by 16' long area between 2 retaining walls. The retaining walls extend 90 degrees out from the basement wall. I want the terrain to be 80" below 0 ground level at a pair of 3' swinging basement doors, and slope up to 0 ground level at the outer end of the retaining walls. This will create a sloped terrain area where I can back my van down to the doors for loading. I have created terrain elevation lines from one wall across to the other wall, but then as soon as I edit the lines to change their elevation and create my sloping ramp, the lines extend out across the entire terrain perimeter. How can I confine them to my ramp area only? Drawings attached. Thank you.