A tip I picked up from an Autodesk University class was to use restrictions on surface triangulation length to help create a surface boundary. This is useful for Existing Grade (EG) and Finished Grade (FG) surfaces; surfaces built with Point, Survey Figures, or Featurelines. However, it does not work for surfaces built from a corridor model. It’s a real shame too, because I think it would be most useful for corridor surfaces. So here’s the trick…
In the below image you will see a FG surface that cuts back on itself. When the computer triangulates between the Featurelines it covers a couple areas that we don’t want to have graded during construction. This is not uncommon around the parameter of any EG or FG surface.
What we typically do to fix this is draw a polyline (pline) around the surface where you want it to cut-off and add that as an outer boundary to the surface. This tick will help you create that outer boundary pline.
Notice how long the triangles are in these two areas. With Civil 3D you can limit the maximum triangulation length temporally to help you generate the outer surface boundary. Right-click on the surface and select ‘Surface Properties…‘. Then go to the Definitions tab and expand the content under ‘Build‘ (see below).
After setting the maximum triangulation length, rebuild the surface and edit the Surface Style to show the surface ‘Border’ component.
In the image below, notice how the triangulation / surface doesn’t span across the unwanted areas any longer. Next, use the Surface–>Extract Objects from Surface command to create a pline from the surface Border that we turned on in the step above.
When using this command it is key to un-check everything BUT ‘Border‘ and to select the border you want to extract.
Finally, edit the surface style again and turn off the Triangles and Border. Then turn off the maximum triangulation limit and using the pline you just created, add it to the surface as an Outer Boundary.
[Note: This will NOT work if your surface is built from a corridor model in any way. Even a small piece coming from a corridor model will cause the surface to ignore the maximum triangulation setting.
There may also be more manipulation you need to do with the outer boundary pline created, but this gives you a good start.]
THAT’S IT!, thanks and I hope this helps…