Development is only ever as good as the tools. We have various development applications that allow for the map to be very rapidly put together. Its based on a similar principal as the heightmap
Where the heightmap creates the terrain, we then have overlay maps, that we use to locate, trees, hedgerows, roads, buildings, etc.
Each feature is on their own map, that simply takes about 3 or so hours to draw up. Once thats done, the hard work begins. As we will have a very rough cut of the entire map inplace in about 20 odd hours work.
Then we place the map on a server, using the sandbox we all login, and systematically go through the map, adjusting the buildings locations etc. Applying properties etc and genearlly adjusting the level.
We have around 15 developers working on the map, so the extensive area suddenly becomes far smaller. Its a painstaking process, but it means that everything is done right.
Automation helps to get the shell of the level together in a very fast and rapid way. So its not as long as say creating the entire level in Unreal Ed or Hammer.
That said, assigning the properties is very very slow process, I'll be writing a script that will do this automatically base on filename structure. But thats still a long way off.