‘Illegal (self-intersecting or non-closed) boundary polygon’ error

This error can be thrown when adding certain boundaries to surfaces, and is caused by the polyline being used looping back on itself.

To fix this, there are two options:

  1. OFFSET the polyline out, then back inwards by the same distance to return it to its original location. Offsetting the polyline like this should remove most self-intersections.

  2. Find self-intersections using the FINDSITEOVERLAPS command.

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Finding the Civil 3D version from a .DWG file

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Find Self-Intersections