Manage Surfaces, Part 1

Many of my clients have come to me with massive projects that they want to work with in Civil 3D. I can see the sweat bead up on their brow as they ask, "Can Civil 3D handle this much data?"

The answer is most often, "Yes." That is, as long as the you're willing to manage some of your bulky data in ways best for Civil 3D. Often, the single biggest piece of Civil 3D data in large projects is the existing conditions terrain model. This month, we'll take a close look at how to tame those monster surfaces and make them more manageable.

An Aerial Example
As an example, let's look at this 5,700 acre project. The existing ground aerial contours and spot elevations alone take up 7.3 MB of space a drawing file (in Civil 3D 2008 with the ISAVEPERCENT variable set to 0).

Aerial contours for a 5,700 acre project site.

Creating a single terrain model from these 1,400 contours (with the default weeding and minimizing flat area settings), generates a surface of 649,000 points. The surface caused the drawing file size to balloon to 45 MB. Many new users become a bit anxious when they hear about things like this. "Does this mean every drawing file that needs to reference that surface will be that big to start," is the common plea.

Luckily, the answer is, "No." Using data shortcuts or working with Vault lets you store data (such as surfaces) in one drawing and reference them in others without adding all that overhead to the other drawings. For example, when I created a design drawing file from the stock _AutoCAD Civil 3D (Imperial) NCS LDT.DWT template, the empty file had a disk size of 521 KB. Most of that size comes from the styles, blocks, and other data resident in that template. Now, if I shortcut in the existing ground terrain model to this drawing, the file increases to a whopping 526 KB. That's right, it adds a whole 5 KB, with 2' and 10' contours displayed. So, we don't have to worry about that huge existing ground surface making other project drawings enormous.

Breaking It Down
This is good news for folks working on massive projects, but it isn't the whole story. Although shared data does not become resident inside drawing files, Civil 3D still needs to "think" about that large surface whenever you are performing certain operations, such as when cutting profiles or calculating surface volumes. In many cases, processing massive surfaces like this is just too much strain for the average PC to handle -- even those with the recommended 2 GB of RAM. Realistically, you may very well need to break up that surface into smaller, more manageable pieces. How small? I suggest keeping surfaces generated from contour points at 50,000-75,000 points or less.

Many new users often bristle at this suggestion. "But we're working the whole thing as one phase!" or "But the road travels the whole length of the surface!"

Although this may sound strange, with Civil 3D this kind of arrangement is more than possible -- in many cases it is a much more efficient way to work.

Take for instance a 10,000' road project that crosses over a 100,000-point surface. First, we can break up this surface into two halves that exactly abut one another around station 50+00. These surfaces can live in separate drawings or in the same one if we preferred.

We could make another drawing to hold a single alignment that runs the entire length of the project (no need to break that up, unless we wanted to allow for multiuser editing). If we wanted to have a single, continuous profile view that showed existing and proposed conditions, we could create one right in the alignment drawing after referencing in both surfaces. If we found that drawing a bit slow to work in, though, we could always create multiple profile drawings, each containing a profile view displaying a different section of the road. Each of these profile drawings would only need to reference the alignment and surface that would be displayed in that portion of the profile. The same holds true for corridors -- you can create multiple corridor objects in separate drawings that only reference the portions of the alignment, profile, and existing ground surface that the area they are being used to model. For those corridor portions that cross from one existing ground surface to another, different corridor regions can be configured to daylight (grade to existing ground) on different surfaces in the same corridor.

Next Time
Once you buy into this idea of breaking up terrain models into manageable pieces, some methods of breaking apart terrain model data are better than others. In our next column, we'll explore some of the details that go into creating multiple adjacent surfaces from the same set of surface data.

For more information, read Autodesk's document "Best Practices for Working with Large Data Sets."