descr_geography.db is the file that determines which ground textures are used in the battlemap, which colours are used for groundtypes in distant views and various factors concerning the elevation and shape of the battlemap.
It appears to have been originally based (by CA) on the contents of descr_geography.txt however unlike the descr_vegetation.db in RTW it will not re-generate from the text file if deleted, the text file is not read by the game and is only useful for record keeping purposes.
The file is located originally in RTW/data only, it can be copied into BI or a mod-folder and any changes made to that version will effect BI / the mod solely.
 Problem in M2TW
With earlier patches of M2TW a timestamp issue meant if you unpacked your M2TW files you had to delete completely the unpacked version of descr_geography_new.db and the descr_geography_new.txt file so the game refers back to the packed version. Failure to do this resulted in a CTD when attempting to start a battle. With the current patch you can simply delete the descr_geography.db file and use
rebuild_geography_db = true
in your config file and a new db file will be generated from the txt file.
 Editing descr_geography.db for RTW
The file can be opened with a normal text editor but attempts to edit with a text editor will result in a non-functioning game. The file can be edited successfully with a hex editor. The main source of information on editing this file is the Guide to Battlemap Modification tutorial by Muizer.
 Re-pathing for mod-folder
The texture file paths are listed in the db file as path:
when using a mod-folder if you put revised battlefield texture files on that equivalent path in the mod-folder they are not read and the game goes directly to rtw/data/terrain.
However you can relatively easily edit the path so it looks for a name not present in rtw/data/terrain. To avoid problems with this file the path length should remain the same. Putting files in my_mod/terrain/battlemynew/ folder so path length stays same and entry becomes:
seems to work without messing up the .db file and picks up the revised textures from the mod-folder.