I tried something a little different in the second try this morning.
The following process took me 12 minutes:
1. Go to Terrain.party and generate the heightfield for the area around Remagen.

2. Load the heightfield into L3DT Pro.

3. Look at the mesh in L3DT and make adjustments.

4. Export the file as .OBJ format, and import it into AC3D.
