I called Bentley and they told me that Openroads uses terrain extractor to interpolate elevations inside the triangles on a digital terrain model to create a surface, but I don't understand there method says to interpolate their elevation by first finding the face that the point is on? Are they trying to get over to the point of interest? The point is inside the triangle? How do you determine which face to perform work on, if they are trying to find a way over to the point. I would also like to know how to do planar interpolation to solve for new elevations inside the triangles?