After a recent conversation with a longtime friend and Autodesk University Instructor, David Garrigues, I decided to post his interesting insight and content from one of his previous Autodesk University classes, on FG Corridor Surfaces. The illustration below displays how pasting EG and FG to together to create a composite surface does not produce the expected or desired result. These results are evident only upon creation of an entirely enclosed corridor, like in a Subdivision Layout, Pond, enclosed Berm or a moat.
Separate EG and FG surfaces
Desired Composite Paste
Instead of a clean paste, as expected with a continuous contour, you will get a void in the enclosed area. This results when using two boundaries: outer and inner (hide).
Paste with VOID outcome (contours only style) Paste with VOID outcome (contours & tin style)
Or you might get straight triangulation across the gap, flattening out the EG conditions.
Paste with FLATTEN outcome (contours only style)
Paste with FLATTEN outcome (conturs & tin style)
Both conditions created above result in false Contours and Earthwork Quantities.
To create a single boundary that encloses the entire corridor design with ONE (1) polyline. You need to play with where to cross in between corridor samples to loop in the void areas and wrap around the outer perimeter.
Upon creation of a single continuous boundary, assign it to the Corridor Surface, and the Composite Surface will build appropriately.
Correct Desired Composite Surface (contours only style)
Correct Desired Composite Surface (contours & tin style)
Hopefully, these steps will clear up any issues you may have seen in the past when building corridors and composite surfaces.