Best Practices for 2D/3D sketches based off uneven "ground" surfaces in point clouds

[throwaway_222223]

We're wondering how to create sketches for gates that have to open and close across concrete driveways that are not only sloped toward the street, but are also sloped side to side. In other words, the driveway is not a simple plane sloped in one plane, but in two (front to back & side to side). 

I tried adding cloud points to the point cloud part file but the cloud points disappear when we try to create a 3D sketch. The 3D sketch would be helpful for at least being able to measure the slope across a given section of the driveway.

Purple is slope toward street, magenta is slope from right to left, green is a dip in the driveway.

Would be nice if we could do a 2D or 3D sketch of the posts of the gate, and have the bottoms of the lines for the posts automatically trim themselves to the surface regardless of how uneven it is. Is that a thing we can do easily, or would we just place the lines and eyeball it to trim them manually? I know we can place cloud planes or place 3 cloud points and create a 3 point plane from that, and then maybe we can try to constrain the bottom end of the vertical line for the posts to these planes.

Any ideas?

[Frederick_Law]

The gate have to clear highest point on the ground.

Wheel with spring will travel up and down on the ground.

Unless gate drop down like garage door.