Manage Surfaces, Part 2

Civil 3D's intelligent, dynamic terrain modeling system can overwhelm even the most high-powered PC when dealing with massive terrain models. In last month's column, "Manage Surfaces, Part 1," I discussed a rationale for breaking up large terrain models into smaller pieces to make a design project more manageable. This month, I offer some specific suggestions for the actual carving-up process. Below I describe each section of a surface as a tile -- this is my descriptor and not a Civil 3D term. A tile surface is a normal Civil 3D surface in every way, just one limited to a boundary of your choosing.

A large surface can be defined as one where surface operations become too painful to wait for or one that causes out-of-memory errors. Personally, I recommend that existing ground terrain models generated from contour data shouldn't exceed a maximum of 50,000 to 75,000 points.

Exclude Extra Info
The first step is to exclude what you don't think you'll need. That may sound obvious, but it's still worth mentioning. For example, one large project I helped with consisted of a series of access roads spread over a 5,700-acre site.

Aerial contours for a 5,700-acre project site.

We estimated that we wouldn't be doing any site work more than 1,000' away from the access roads. Therefore, limiting the surface to that area only cut the surface down to one-quarter of the original surface points (from 649,000 to 166,000).

The total area we expected to impact with project roads. This will be divided into three tiles for convenience.

But, here's the problem -- how do you limit the surface data? Adding a surface boundary may not buy you the performance you need. My experience is that some operations are not sped by having a surface cut back with only a boundary. You very well may need to completely exclude data you don't want from the surface definition.

This operation can be tricky with surfaces created from aerial contours because contours can extend across the entire site, and some won't cross your boundary at all. If you've never done it, limiting contours to a set boundary can be an all-day affair with AutoCAD's Trim and Erase commands. But, the AutoCAD Map Boundary Trim command (type in Maptrim at the Command line) can save the day. After launching this command, you see the dialog box below. After selecting the polyline you want to use as a boundary (with the Select button), assign the other settings shown and all linework not on frozen or locked layers is trimmed to the boundary. Furthermore, all linework outside the boundary not on frozen or locked layers is erased. It goes without saying that this is a powerful command, so be careful with it!