A DEM is a digital elevation model. You get the space shuttle flying over the earth with a radar on board and it measures the height of the ground in sweeps so that there is a 3D terrain model you can extract from it. Since it provides the height for the map we can get the sea and coast outline and get some scripts to shade it all in any way you want. You can go as far as to create a climate model for it and use that to determine where the rain falls, where the rivers flow and what vegetation would grow there. Normally, making a fantasy DEM is quite hard but its fairly easy to get one of a real place from NASA data if you have a bit of programming skills or the data format matches a tool which understands whatever format you have. The DEMs are out there in a variety of formats so often you need to code a converter.

Jax, ill run my script a bit later on when I have some spare time and you can mess about with the resulting images if you want. I'm no artists - famous, dead or otherwise