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AutoCAD

Manage Surfaces, Part 2 (CAD Clinic: Civil 3D Tutorial)

30 Nov, 2007 By: Mike Choquette

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Details about making large surfaces more manageable in Civil 3D 2008.

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.

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).

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!

The boundary and contours after using the Maptrim command.

You can repeat this process for each drawing file that will hold a tile or section of the existing ground surface. The contours above created a surface of only 58,416 points, and its drawing file was only 4 MB in size (down from 649,000 and 45 MB for the entire site). A design drawing then can reference only the existing ground terrain that is needed for its profiles and corridors through data shortcuts or Vault references. By referencing only the data necessary, processing time is reduced dramatically. In the above project, we understood that there was the risk of having to move the roads and recreate these surfaces, but for us the 75% reduction in data volume was worth it.

A word of caution though -- for those design drawings that must reference more than one surface tile, check the interface carefully. In most places, the surfaces will line up reasonably well, sometimes with contour data it's possible to have slight discontinuities between each surface. Some surface editing may be required to adjust for any issues.

Create a Single Surface
One advanced technique for ensuring that they line up is to temporarily create a single surface of the entire project site and then convert the tile boundaries to feature lines and project them to the surface. These 3D tile boundaries can be used as both breaklines and outer boundaries in the individual tile surfaces, ensuring that the tiled surfaces line up correctly.

Aerial contour data often includes AutoCAD blocks or text at elevation representing spot grades. As long as they have a valid AutoCAD elevation property, these objects can be added to a Civil 3D surface as elevation data through the Drawing Objects heading under the Surface Definition in the Prospector tab.

Keep in mind that surfaces don't update if AutoCAD blocks or text objects are moved or adjusted. In the big picture, this procedure probably is for the best though, because surveyed spot elevation blocks rarely need to be changed. Note also that Civil 3D 2008 has a new utility that automatically moves blocks to the elevation value of one of their attributes or moves text to the elevation value of the text object. This utility can be invaluable if spot elevation blocks are at elevation 0 but have valid elevations as attributes or text values. Find these two routines under the Utilities area of the Surfaces pull-down menu.

Final Suggestions
When you add points and contour data to the same surface, be sure to add the points before the contours, because some of the Civil 3D 2007 and 2008 Minimize Flat Areas options add interpolated points between contours to flesh out the surface. If you use these options, you want this addition to happen after the actual points are already included in the surface definition so they can be taken into account.

On the other hand, if you'd rather not break up your large contour-based surfaces at all, you may get improved performance out of Civil 3D by reducing the complexity through the options in the Add Contour Data dialog box. For example, increasing the Distance and Angle values under contour weeding results in a slightly less accurate but smaller and more manageable surface. Also, disabling the Adding Points options under Minimize Flat Areas circled below can prevent any additional points from being interpolated between contours. Please see the Civil 3D Help system for more information about these options.

To summarize, Civil 3D can work with very large datasets, even massive terrain models. Many settings and methods make this possible, but keep in mind that it may be best to break up large terrain models into smaller pieces. For more information on this important topic, please refer to Autodesk's "Best Practices for Working with Large Data Sets."