Andantonius: Strangely, I was looking for cartography resources when I found BioRust. There are some dedicated programmes for doing this sort of aerial view map, primarily for gamers, but I can see other uses.
The three main programmes I've found are
Campaign Cartographer ($40) which is quite professional and has lots of companion products,
Fractal Mapper ($40) and
AutoREALM, which is free but unfortunately the results look poor.
I am sure it would be possible to assemble a good library of backgrounds, tiles, and brushes in order to make one's own maps in PS. But I wonder if it's worth the time given the dedicated tools these other software packages provide?