Breaklines

I’d have to say the most misunderstood term in land development is “breakline.”

As far as I’m concerned, an exercise in grading needs to begin with an exercise in terrain modeling. Whether existing or proposed ground – it’s all about the breaklines.  If you’ve ever had the pleasure of manually creating contours on the board from a grid of points based upon hand-written notes in a field book, then chances are you’ll have a better understanding of how breaklines work (if you do, then you’ll likely remember how to construct a circle through 3 points – the old way). 

Imagine four points that define the four corners of a square.  When connected by lines, any three points define a triangle. You can create 2 triangles in a square, and there are two possible solutions – either the hypotenuse of the triangles runs southwest to northeast, or northwest to southeast.  The algorithm running the math behind the scenes builds triangles, but it’s human intervention that ultimately determines triangle direction.  Terrain modeling isn’t a science, it’s an art; you can’t let the software run wild building triangles, you need to help define their direction based upon your understanding of field conditions.

What do breaklines do?  Breaklines effectively tell the terrain modeler “do not cross this path with a triangle.”  Take a look at the following images, notice the triangles (in the image to the right, the edges are covered by the yellow breakline):

But take a closer look at the shots, they’re CL (centerline) shots.  You and I know the typical roadway has a well defined crown (unless you’re in Texas where it’s common to see a well defined parabolic crown), but how does the software know?  It doesn’t.  How do you tell the software to create a crown?  You create a breakline.  Notice the orientation of the triangles after the addition of a breakline to the model.

Notice how the contours more accurately represent ground conditions? That’s what terrain modeling is about – you’re a triangle traffic cop.

So it sounds like I’ve nailed existing ground, but what about modeling proposed ground – is there a difference? Absolutely not. It’s still about directing triangles. Sometimes simple polylines work, sometimes we use feature lines, sometimes we use points, sometimes we flip faces or swap edges (telling triangles to direct themselves in the opposite direction), sometimes we use hard core grading objects or corridor models. There isn’t a right way or a wrong way – there isn’t necessarily a right tool, or a wrong tool.

I think often the problem is this: you don’t know what you don’t know. So as you’ve been trained to do (I call it “the way we’ve always done it” syndrome), you contact your local reseller to take a class in grading. And 9 times out of 10, that’s exactly what you’ll get – grading… sites, feature lines, grading objects – how to push buttons. But if the course were instead a study in “terrain modeling,” you’d open your eyes to a whole new way of approaching digital land development, of which “grading” is a small part.

Civil 3D Library
Looking for Grading & Terrain Modeling tutorials? Check out The DTM Branch in the Library. These downloads aren’t software demonstrations. They were created for clients using AutoCAD Civil 3D in the real world. Registration is required.