I'm not sure how to interpret your qwestion so i'll tell you instead what i'm doing (yeah, and Misantrop needs it too)
Each tiles can be elevated one step, that means we have two digit tiles, one level of slope. There are 4 corners so we need 2^4 = 16 different slopes for each tile, here's an example:
http://www.geocities.com/kedasu/isotiler.jpg
Then when the horizontal corners are in a different elevation, the stretch will be done by left and right part independently, creating a line in the middle
And when the vertical corners are in a different elevation, the top and bottom are stretched independently.
WEll that's the kind of elevation you have in TRansport tycoon or Tiberian Sun, In settlers you will have 4 levels of elevations and the last and first one starts a slope up or down. That means 2*3^4 = 162 types of slope (yeah they are hexagonal so you have two types of triangles)