I've always used copies of ADTs from QA_DVD map (they can be found in lichking.mpq). The've served me well - on about 10+ maps, even while someone told me its not a good idea to use them. Well, I've never seen any reason why I shouldn't do so.
I just copied 1 original file and separated the files like this:
MapName_x tile_y tile.adt
x and y are ranging from 4 - 14 coordinates.
When I open noggit, I only see emptiness, with no adts in there, just a blank world.