The terrain heightmap was imported from GeoControl 2 which I'm addicted to again.
The ground is just a very simple 256x256 procedural Vue terrain object that I've applied a GeoControl 4096x4096 TIF heightmap to both its altitude and bump settings. The terrain was then scaled to 500ft high.
I probably should have made it 1000ft to get a better scene scale. This GeoControl 2 video shows how to make similar terrain.