It's created by Terragen according to my settings, largely reduced in polygon count by using the method mentioned in the wiki and then fixed with some ingame editing which is available in the desktop version of the game. In addition, it's modified in code too by placing houses and fences and stuff, which all flatten the underlying terrain. The splatting texture has been drawn by me based on the height map created by Terragen.
The geometry is more or less final, but the splatting texture isn't. It's missing a lot of additional locations, tracks etc.
Edit: And about the view of the map ingame...it's a simple image created by a stand-alone program that renders the whole terrain from above at once.