Civil 3D feature line offset and mid-ordinate distance

Civil 3D Rule: When adding Feature lines as breaklines to a surface, use a "mid-ordinate distance" smaller than your smallest arc offset.

Louisa Holland blogged about "Why does it seem like Civil 3D is ignoring parts of my breaklines?" I might add:

Why does the Civil 3D Event viewer keep telling me a feature line "Breakline not added...crossed a breakline at...."? I can plainly see the breakline does not cross any other breakline. Is there a phantom line? Is Civil 3D having an error? Do I need to audit my drawing? No.

Civil 3D Rule: When adding Feature lines as breaklines to a surface, use a "mid-ordinate distance" smaller than your smallest arc offset.

This was happening to me, and as Louisa's blog hints, the problem I experienced was that I was offsetting feature lines (for the MAG/Arizona curb faces we do at Hubbard Engineering) 1 inch horizontally. But my "mid-ordinate distance" in adding the feature lines as breaklines to my surface was 1.00' (12 inches). This meant that eventually two adjacent arcs were going to get segmented (tessellated is the technical term) in a way that made them cross each other.

Civil 3D has to break arcs into segments to use them for surfaces. This of course introduces an error. The maximum error distance is called in Civil 3D the "mid-ordinate distance". If you let the mid-ordinate distance be greater than the offset distance between two arcs, their segments (tessellations) may (will eventually) cross. The solution is to set the mid-ordinate distance no greater than the closest distance between adjacent arcs.

Of course you may want to also change the default value. You can do this in AutoCAD 2010 as E. Chappell showed me. See Part 2.