I found a great free resource the other day for people who want/need to work on large terrain models - Tangram Heightmapper
Once you save a height map of your location, you need Thom Thom’s Bitmap to Mesh extension to convert to 3D surface. Note make sure to reduce image resolution to 500-1000 pixels as anything bigger may not import as it tries to generate so much geometry.
Here’s a quick example of Yosemite to scale (fyi that’s a person on top of Half Dome there):
Cheers.