Civil 3d – Surface Breaklines: Standard or Proximity


Assigning breaklines to your surface definition is probably the most important step in achieving a correctly triangulated surface.

We often find users questioning whether they should use standard or proximity breaklines.  In this blog we will discuss the main differences between the two.

To simply put it:

A standard breakline is one that has elevations assigned to every vertex (XYZ).  This means you can only use AutoCAD or Civil 3d objects that are fully three dimensional, such as; survey figures, feature lines, 3D polylines and lines.

Proximity breaklines do not need elevations at their vertices.  This means you’re allowed to use an object that is drawn flat at 0 elevation.  The elevation is instead extracted from the nearest TIN point elevation or corresponding COGO point. (hence the term proximity)

Here are some facts about both types:

  • Standard Breaklines
    • Support curves through the mid-ordinate distance supplemental factor.
    • Perfect for automatically generated survey figures.
  • Proximity Breaklines
    • Commonly defined from 2D polylines
    • Curves are not supported (arcs are treated as if they’re straight lines).

So to get the best surface definition possible, make sure you use the proper breakline type, for your application/surface that you are currently working on and trying to update. After all, having a correct surface is very crucial.