In terms of stitching arbitrary height maps may I point you at some work done by Howard Zhou who has been working on this sort of thing for a while...

http://terrain.cg-arts.org/forum/ind....msg821#msg821
http://www.howardzzh.com/research/terrain/

To keep it simple, I believe he is finding the closest path between two maps and cutting along them - a bit like photo stitching I think. I reckon there is a lot more to it tho. Its unbelievably awesome actually. Anyways there is a world machine plug in to do it.