Represent intersection between surface and terrain

Hi!
I would like to know what is the way to represent the intersection between an arbitrary inclined 3D surface and the terrain. For example, imagine that a slopped triangular surface intersects the terrain (ASTER for example) and I need to represent that intersection (if any) by a line (similar to contour lines).
Thank you in advance!
Thank you in advance!
Answers
Subtraction
Create contours with value "0"