I finally finished the heightmap. Heightmaps following this tutorial are VERY slow to make if you don't have high processing power (took me 3-4 hours to wait for SketchUp to finally charge it all). anything like an intel-i7 or higher I guess should do fine.

Here are some shots:

Finaly almost done.PNGDone.PNG

I'm doubting between making the buildings in another file and then importing them to this one or making them directly on this file even with the slowness of moving around...