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    by David Lewis

    Boundaries are typically created from closed 2D polylines to limit a surface triangulation internally or externally. A boundary can contain lines and arcs. As the surface has been generated based on triangulation, Civil3D uses mid-ordinates to calculate where triangles interact with the boundary. Users also have the option to use destructive or non-destructive breaklines.

    Boundaries are added to a surface by navigating to the surface definition in Toolspace prospector tab, right clicking boundaries and selecting ‘Add.’


    Boundary Types:

    There are 4 types of boundaries; Outer, Hide, Show and Data clip.

    • Outer: Hides or excludes data outside its edge.
    • Hide: Hides an internal portion of a surface such as water bodies and building footprints.
    • Show: Displays a portion of a boundary within a Hide boundary such as an island in a pond.
    • Data Clip: Acts as a data filter.

    Note: An outer boundary should be amongst the final items added to a surface.


    The mid-ordinate distance is the distance between the midpoint of a chord and the arc. The smaller the mid-ordinate the closer the triangulation surface is to the original arc.

    Boundaries with Destructive and Non-destructive breakline

    Users can control the relationship between the boundary and the surface by determining whether the boundary should be applied as either a destructive or non-destructive breakline.

    • Non-destructive breaklines: Extends triangles exactly to the boundary line and preserve the original surface levels at their intersection with the boundary. 
    • Destructive breaklines: Removes triangles that extend beyond the boundary. This is typically used when the boundary does not represent a hard edge.