Overhang Correction to Repair Wacky Corridor Surfaces

So I had heard about this idea of corridor overhang correction that is new for 2008, but I hadn’t really run into anything that required using it until two weeks ago. I was working with a really terrific group of guys and gals that design dams and spillways. We built some pretty simple but powerful assemblies using generic links.


All went well until we built the surfaces….

image image

Read on to find out what was happening.

If you want the whole scoop on how corridor surfaces are built using points, links and feature lines, you’ll have to buy the book and check out Chapter 11. However, in a nutshell, what is happening here is that since we used subassemblies that have points are all coded “top”, the surface doesn’t know how to connect the point codes. They all seem the same to the surface, they are all P2 in this case. It simply doesn’t care about the connection order right now. It makes its best guess and leaves it up to us to figure out what is wrong.

Adding your links as breaklines and additional feature lines may help in some cases, but when using generic links, every single point is coded P2, so it isn’t very helpful here.

If you go into Corridor Properties, there is an overhang correction option. Try the choices in this box and rebuild your surface to see if it makes the right difference.


In this case, the Top Links choice worked to fix the surface.


For Nick’s explanation of overhang correction check out this post:



A lot of people comment that this all seems like a lot of voodoo sometimes. There are so many settings and so many tabs in the corridor properties box, how will you ever figure out what is right? This is how I feel about it. If your resulting surface comes out looking appropriate based on your engineering skills and judgment, it isn’t necessary that you touch every single tab and setting in the corridor properties dialog.

Civil 3D isn’t instant design-just-add-corridor.

It’s a tool that relies on your judgement to tell it how to do things. As an example, I’ve build hundreds of corridors over the past two and a half years, and it has only been the past six months that I have gone looking for ways to improve, refine and sharpen my corridors using the advanced options such as adding links as breaklines, adding additional feature lines as breaklines, building custom code sets, etc. And I will admit I have NEVER used the Slope Patterns tab.

So take heart. Start simple and don’t be afraid. Built a corridor, examine the surface, figure out how it is working and what you’d like to refine. Then go looking for the settings to fix it. If you try to master the corridor properties box before you ever build your first corridor, my coming-soon son Prospector will be in college and you won’t have even made your first baseline.

One comment

  1. Oh, this…is…Awesome! I found this setting by accident, but that’s exactly what my corridor needed. Thanks for the tip!