I found this ancient thread, and wanted to bring it up to the top again, as it contains a fascinating tidbit that all mapmakers should know.

Quote Originally Posted by Caliban
I'll have to get back to you on the heightmap prob. I'll find out the procedure, or maybe one of the more experienced campaign guys on here could help.

If you just want to flatten the campaign map so its not so hilly, you can edit:
\data\descr_geography_new.txt

search for: height_scale 1.0

and change the 1.0 to a smaller value. This determains the height range of all the terrain. Changing it to 0.5 will effectively halve the total height range of the ingame terrain everywhere. 0.1 will be extremely flat. This will only effect the campaign map, and not historical maps or custom-made maps.
You will also need to delete \data\descr_geography_new.db I havn't tried any of this stuff in the retail build so there may be some issues with duplicate data (such as pack files etc) that cause your changes to not take effect. Normally deleting those pack files or databases make your changes re-build on next game load. I Hope this one works for you :)