Even with a level 1 map_heights and a completely zero map_roughness it still generates a rolling terrain.
Which isn't very surprising. The map_heights.tga cannot give enough information to make an interesting battlefield. So on top of the rough height differences a second layer is generated. I guess that certain climates produce less hills than others.

I thought that there might be a text file that would determine the relevant heights, but the landscape on a particular tile is always constant - which suggests that it's referencing a graphic.
A noise effect in PS is done by a formula and only by adding some random factor it changes each time. I think CA removed that random factor uses a formula that always produces the same results based on what the campaignmap looks like. The text files does hint at what variables are being used. There are several text files that deal with how the map are being generated. In the past I did some testing, but I never could find anything working.
There is a text file which does give you some influence; descr_terrain.txt in the base world/maps/folder:
Code:
heights
{
	min_sea_height  -3122.256
	max_land_height  7511.272
}
roughness
{
	min  50.000
	max  200.000
}
fractal
{
	multiplier  0.500
}
If you chance the height section then you can influence the range of height differences on the stratmap and as a consequence also on the battlemap. Reducing max_land_height to 1000 gives a much flatter landscape with the same map_heights.tga. I guess that roughness and fractal deal with the second layer of height differences on the battlemap.


Another question is where does the game get the information for the background on the battlefield from? The battlemap tile itself comes from the textures in the data\terrain\battlefield\, I tried changing the textures used on the aerial map with no success.
If you mean the very low resolution groundtextures bordering the battlefield then forget it. I have tested every texture and looked through any relevant txt file but I couldn't find anything. I suspect that the engine generates a few colours from the ground textures and stores them in descr_geography.db
I had hoped that the database could be generated just like with descr_vegetation.db but it doesn't, or at least I don't know which files need to be removed to trigger it.

Cheers,
Duke John