For those who might not know (and I haven't seen it mentioned here anywhere), toggle_terrain is a command available for modders through the medieval shell command. Press "~" in the campaign map to gain access to the shell.
toggle_terrain allows you to switch between the various modes of viewing the terrain. All of the terrain .tga files in your /base folder have a 1-to-1 correspondence with each other. This means that the regions file exactly corresponds to the features file, and to the weather file, etc. So what you can do is see those files exactly how they apply in the game.
The most useful command for me so far was "toggle_terrain region", which allows you to see exactly how the descr_regions.tga file results in province borders in the game. It helped me directly to correct some problems in the teutonic map, and it will take me longer to concretely explain its usefulness than for you to try it out and immediately see its power.
Other commands, I believe, are toggle_terrain features, which shows exactly how rivers work, how their starting point affects them, etc.
You can see all of the possible arguments, when you type ?toggle_terrain. I highly suggest you try them all, because you'll be really surprised with what you find. Type toggle_terrain with no argument, to return the game to its normal look.