Forum Post: RE: How do I create a composite surface that represents the LOWEST of a collection of stacked surfaces? (V8i SS3)

Tim, Glad it worked for you! FWIW, about 16 years ago, I was asked to create the following video to help explain what the imaginary surfaces are. Understanding Part 77: Civil Airport Imaginary Surfaces It looks a bit dated now, but it was one of my first animations in MicroStation.