This modifies the map for the vanilla campaign. Ground has been flattened and some forests removed.
This is the vanilla campaign map with all ground flattened (there are still hills but not ridiculous cliffs everywhere). Also it has all the forest that armies can move through removed, so there will be no more forest battles that you can't see your men in.
The download is just a zip with map.rwm in it. Depending on the mod you use, it needs to go into either data/world/maps/base or data/world/maps/campaign/imperial_campaign.
Of course if you are using a mod, you have to start in the mod's folder and not the default data folder.