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Autodesk Land Desktop Tutorial: Building A Surface From 3D Faces

By Rick Ellis, Cadapult Software Solutions, Inc., June 5, 2003

We have all probably received a drawing at one time or another that contains 3D faces representing a surface. Now your job is to convert these 3D faces into a Land Desktop surface that you can use for creating Profiles, Sections, Contours, and a variety of other things. The new, and best way to translate a surface from one software package to another is to use the LandXML import/export format. However, not everyone has new enough versions of their software to support this solution all of the time. So we will look at how you resolve this issue the old fashioned way, by working with the 3D faces.

Your first thought might be that this is easy. In the Terrain Model Explorer you create a new surface. Then under the new surface you look at your options for TIN Data and right-click on Point Files. Under Point Files there is an option to Add Points From AutoCAD Objects. This will sample the points at the vertices of all the selected 3D faces. The problem here is Land Desktop is only sampling information from the corners of these 3D faces, not the association between the points. So when Land Desktop builds the new surface it does not have any breakline information and will just connect the points by their proximity to one another. Therefore, it is very likely that your new surface will not look exactly like the 3D faces that you started with.

So, what can you do to get an accurate recreation of this surface? You need to treat the 3D faces as breaklines. Then it will triangulate exactly like the original surface. The problem is that you can’t create breaklines from 3D faces. Somehow the 3D faces need to be exploded. The next problem is that AutoCAD cannot explode a 3D face. There are several lisp routines that you can find on the Internet and download for free that will explode a 3D face. The one that I use is called 3dfto3dp.lsp and can be downloaded from Dotsoft. This converts your 3D faces to closed 3D polylines. Now the AutoCAD Explode command will explode the 3D polylines to 3D line segments. Finally, something that you can use to create breaklines.

Not so fast. If you look closely at your drawing you will find that you have duplicate lines everywhere. This is because all of your original 3D faces overlapped each other. You need to use the Drawing Cleanup tools in Map to delete the duplicate objects. Select <<Map>> <<Tools>> <<Drawing Cleanup>>. Use the selection options to select all of the 3D lines. Then set the cleanup options to delete duplicate objects.

Now that you have individual 3D lines you can add them to your surface as breaklines. Then build your surface from the breakline data and you will have an exact copy of the original surface.

About the Author

Rick Ellis is the co-founder of Cadapult Software Solutions, Inc., a member of the Autodesk Developer Network; and author of several AutoCAD training manuals, including “Digging into Land Desktop.” He teaches classes focusing on AutoCAD, Map, Land Desktop, Raster Design, MXRoad and MXRenew for skill levels ranging from beginner to advanced users. Consulting services range from installation support to the implementation of standards.

If you have any questions about this article or for more information you can call Rick at (503) 829-8929, email rick@cadapult-software.com, or visit the Cadapult website at www.cadapult-software.com

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