First I need some reference. I say how about the Hoover Dam? That gave me lots of reference including this illustration of the basics.
After drawing a sketch I now block out the basics.
Had some strange problems with importing flight files that turn the normals inside out in the screen display. Anyway here is the road and Dam layout. Roughly modeled after the Hoover. I added a little water on the other side of the dam so you will be able to drive over it and see both sides. Put a hair pin turn in the road to the dam too. Now I need the guard shack below. I have one but its 1200 verts and like one about 250 virts.
Starting to paint the topical Map. I have it at 8000 pixels x 8000 pixels and then reduce it to what I need. That way I save my high res work and if needed II an reduce as necessary.
Checking the map on the mesh often to see how it fits the terrain.
Continuing to paint out the terrain.