For a while now, I’ve been preaching how to add contour data to my classes and nowhere have I ever seen anyone else teach it the way I do. I’ve done some searching online and even the folks over at (formerly do it the same way everyone else does, and they are doing it wrong (sorry).

So, what do I do different then everyone else? First off, let’s talk about what everyone else does.

The Incorrect Way To Add Contour Data

When you add contour data to a surface, you are offered four options for minimizing flat areas:

Adding Contours

As you can see in the image above, three of these options are toggled on by default (1, 3, and4) and one is toggled off (2). Everywhere I’ve ever seen anyone talk about adding contour data says “Use these settings when adding contour data”. Examples:

Even the official Autodesk Best Practices Guide for Civil 3D 2012 says to do it this way:

So what’s wrong with doing it this way? Well, IT JUST DOESN’T WORK! First off, let’s see what happens when you don’t do any minimizing at all. In the following image, you’ll see a group of contours. These are 5′ and 25′ contours.

Original Contour Data

Now, I’m going to add these contours to a surface and I’m not going to do any sort of  minimizing options at all (all unchecked). The resulting surface (showing 5′ and 25′ contours) shows why you need to minimize your flat areas.

Contours Added w/o Minimizing Flat Areas

The surface contours and the data contours don’t match up at all, so we minimize the flat areas. First off, I’m going to show you what happens if you just swap the edges (toggle just option 2 on). When you swap edges, Civil 3D merely  changes the way it triangulates the data.

Flat Areas Minimized by just Swapping Edges

This does a pretty good job fixing the contours but there are still some areas that it just doesn’t quite work. I think it’s because of this that people say not to use the swapping edges method. Instead of swapping edges, the next image shows the exact same surface with options specified by all of the sources linked to above, 1) Filling gaps in contour data* 3) Adding points to flat triangle edges and 4) Adding points to flat edges.
*This is not really a minimizing option and I’m not sure why it’s included in this dialog box. I suppose they had to put this option somewhere and they decided to place it here.

Minimized Flat Areas with Default Settings

As you can see, the new contours match up exactly with the original contour data (within the parameters set by the weeding factors at least). People see this and think to themselves, “Hey! That looks perfect! I’m done!” Well, let’s take a closer look at the surface, I’m going to do this by using a different contour interval. The original data was 5′ contours so I’m going to display my surface with 1′ contours.

Default Options Still Leave Flat Areas

Even though we told Civil 3D to minimize the flat areas, this image shows that there are still flat areas. Well, technically, they aren’t flat. When you use the default options to minimize flat areas, it takes the flat areas and puts a really small slope on them so they are no longer perfectly flat, they are just really really close to being flat. And this is the problem with using the default settings. The surface contours that are at the exact same elevation as the data contours match up exactly. However, between the contours, it just doesn’t do a good job estimating what the surface should do.

The Correct Way To Add Contour Data

Ok, I’ve just shown you that everything you know is wrong about contour data so, what’s the correct way to add contour data to a surface? The answer is suprisingly simple, just use all the options when you add your contour data. For the flat areas, if it can swap the edges, it does. If it can’t swap the edges, it adds the points. The following image shows how this same surface looks with all four of the options turned on.

All Minimizing Options On

It’s not perfect. There are still some flat areas (hence the term MINIMIZING flat areas) but using all the options results in a much better surface than the default options. Hopefully I’ve convinced you to come over to the dark side and use all the minimizing options when adding contour data. Of course, there may be situations where the different options will provide better results (and if you know of any, please let me know what they are) but overall, just toggle them all on.

Unfortunately, there doesn’t seem to be a way to change the defaults used for the Minimize Flat Areas command so you’re going to be stuck toggling on the swapping edges each time you add contour data or use the edit (again, if anyone know how to change the defaults, please let me know).

<edit>Looks like you can now edit the defaults in Civil 3D 2013. Right click on the command AddSurfaceContours in the settings tab and you can change the defaults:

Change the Defaults

Change the Defaults

Thanks to Gunnar on the discussion groups.


Now get out there and buck the C3D system!