## Creating an Accurate Surface From Contours

This morning’s class was about new survey functionality in Civil 3D 2007. This afternoon, we’re working on site grading, something that is much more stable in Civil 3D 2007.

But first, we need a surface to work with, don’t we? Can’t grade without a surface. We have an aerial topo that we can work with, so we’re going to use it to build our surface. In the past, we would have problems with flat areas on the surface – not so much in Civil 3D 2007. So we start with our aerial topo:

Now, we want to create a surface and add this contour data to is. So I create the surface, expand the Definition tree in Prospector, and right click on Contours. I have options now to add supplemental and weeding data, and to Minimize Flat Areas. Yes, this is the theme of this post. For the sake of comparison, I’m going to deselect all the options in Minimize Flat Areas so you can see what happens:

Now, let’s go back and rebuild our surface. If you look, the new contours of our surface do not closely match the original contours:

The red original contours are a little hard to see in the image, so I added some arrows to show you the problem areas. What we’ve got now is a surface, but it’s not very accurate. To make it a little more accurate, I need to go to my surface definition and right-click on Edits – tell me you knew about this. Now I’m going to click on Minimize Flat Areas. Here’s the dialog box that comes up:

You’ll notice that I’ve got options 1, 3, and 4 selected. The reason is that Swapping Edges and Adding Points to Flat Triangle Edges both use the same processing algorithm, but Adding Points to Flat Triangle Edges produces markedly better results.

Now that I’ve minimized my flat areas, let’s check out the resulting surface:

And now we have a pretty accurate surface (well, at least as accurate as the contours we were given). Now we’ve got a pretty good base to start from, and get into the new feature line capability of Civil 3D.

Have fun!