Generating Map Terrain from Image

Hello there,

I am getting started with 3D Level Design and I am looking for some advice on map generation.

As far as I know I should be creating a heightmap, which can be applied to my terrain.

I want to base the world on a real location and so far have been able to get a topographic Map of the area:

Sadly I can’t just convert this image to shades of grey as the topography is just a overlay over an actual map.

So here’s my question:

Does anybody know of a quick and dirty solution to my problem? And if you don’t, do you have any advice how I should approach the issue without hand painting the entire map?

The Image shows about 1/6 of the entire thing so it would take me a while to get down all the little hills and valleys.

I appreciate any help you could give me and thank you very much for your time.