There are a lot of different ways to add data to a surface. If someone gives you data that represents the triangles of a surface created in a different program (such as Eagle Point or one of the Bentley programs), it can be a bit difficult, if you don’t know how. If you’ve ever done this, you may have noticed that the surface you created from this data doesn’t match the original data. Find out how to make your new surface and the data match up!

So, what kind of data am I referring to? Well, if you ever get drawing data that represent the triangles of a surface, they probably come in one of two forms, 3D faces or simple lines. You can add either of these to a surface. You can’t add them as breaklines, contours, points, or any of the other methods, you need to add them as drawing objects.

Adding 3D Faces to a surface

In this example I’m adding 3D Faces because that’s what I got when I extracted the triangles from the C3D surface. Let me go ahead and show you the surface I end up with after doing this. The green triangles in the following image are the 3D Faces I’ve added to the surface and the gray triangles are the new surface.

Wrong Triangulation

If the new surface exactly mimics the old surface, then I wouldn’t be able to see any of the green triangles. I suppose an explanation of why this happens is in order. When you add drawing data to the surface (lines, 3D faces, or polyface), the ends of the lines or vertices of the faces are added to the surface as point data. The surface then goes through the Delaunay triangulation process and creates the triangles.

Obviously, this is not what I wanted so what do I do to correct it? Well, when you add the drawing data to the surface, there is a little check box, “Maintain edges from objects”. If you toggle this on, the surface will add the lines or edges of the face as breaklines. According to the help file:

Maintain Edges From Objects

Specifies whether to define the AutoCAD Civil 3D triangle edges based on the edges defined in the original AutoCAD object. If selected, AutoCAD Civil 3D maintains the edges and does not attempt to optimize the edges. Available when importing points from lines, 3D faces, and polyfaces.

In my example, the area of the surface that doesn’t match the triangles is in an area that breaklines where added to the surface. If I redo the process and this time toggle the “Maintain edges from objects” option, you’ll see I get much better results.

Correct Triangulation

Note, the green triangles are still turned on and visible in this image, they are all simply behind the gray triangles created from the surface.

e the AutoCAD Civil 3D triangle edges based on the edges defined in the original AutoCAD object. If selected, AutoCAD Civil 3D maintains the edges and does not attempt to optimize the edges. Available when importing points from lines, 3D faces, and polyfaces.