Which of the following is not a type of surface boundary?

• A. Proximity
• B. Outer Boundary
• C. Inner Boundary
• D. Breakline

Answer: (A)

Surface boundaries control where a Civil 3D surface exists and how its triangulation is formed. The outer boundary defines the overall extent of the surface, marking the limits of the triangulated area. An inner boundary creates holes inside the surface, removing those regions from triangulation. Breakline boundaries influence how the surface is divided by enforcing a defined line on the triangulation, which preserves ridges, channels, or edges along that feature.

Proximity, on the other hand, is not used to define a boundary of the surface. It’s a different concept used for distance-based considerations or analyses around features, not for bounding the surface region. Therefore, the item not representing a surface boundary type is Proximity.