As discussed earlier, boundaries are a way of defining where the surface is and where it is not. In the example project, you do not want the surface to exist outside of the area that has been surveyed. Why would the surface extend beyond the survey data? If the edge of an area represented by the points happens to bend inward, the lines will extend across the “bay”
(see Figure 1) and will create misrepresented surface data in that area. One way to avoid or correct this situation is to provide an
outer boundary that prevents the surface from existing in these areas.
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| Figure 1: Erroneous TIN lines created across a “bay” in the surface data. |
Another common example of surfaces being where they shouldn’t be is within the shape of a building. It is considered poor drafting practice to show contours passing through a building. After all, the ground surface is not accessible in that location. Another type of boundary, called a
hide boundary, can be used to remove surface data from within a surface, thus creating a void or “hole” in the surface.
Types of Boundaries
The four types of boundaries that you can create with Civil 3D are as follows:
- Outer
- An outer boundary establishes a perimeter for the surface. No surface data can exist outside an outer boundary. This type of boundary is commonly used in most surfaces.
- Hide
- A hide boundary creates a void or “hole” in the surface. Hide boundaries are commonly used to remove surface data within buildings.
- Show
- A show boundary creates an island of data within a hide boundary. An example of a show boundary is a courtyard within the footprint of a building.
- Data Clip
- The first three types of boundaries hide the surface data after it has been created. A data clip boundary is a special type of boundary that prevents data outside of it from ever becoming part of the surface at all. Data clip boundaries are used in cases where a small surface is made from source data that covers a large area.
Follow these steps to add boundaries around the buildings that will remove surface data from within their footprints:
- Open the drawing named Surface Boundaries.dwg or (Surface Boundaries.dwg (METRIC)) located in the Exercise_04 class data folder.
- Click one of the surface contours in the top-right viewport and then, on the Tin Surface: EG tab of the ribbon, click Add Data ➢ Boundaries.
- Enter Bld1 as the boundary name, and select Hide as the type. Make sure the box next to Non-destructive Breakline is checked and click OK.
Tips :The Non-destructive Breakline option creates a clean edge along the boundary by trimming some TIN lines and adding others.
- Select one of the buildings in the top-right viewport and press Enter. You should immediately see a hole appear in the surface shown in the lower right viewport. If you’ve selected a building with contours running through it, you will see the contours disappear in the upper right viewport. It appears that they have been trimmed, but actually, the surface data has been removed from within the shape of the building.
- Repeat steps 2 to 4 for the other buildings, even if contours do not pass through them. Use a name other than Bld1 or the software will not accept your boundaries.
- In the lower-right viewport, zoom in to the area of the buildings and notice that there are now voids where the buildings are located, as shown in Figure 2.
Tips :Remember that contours aren’t the only things surfaces are used for. No matter how you look at this surface, you don’t want data to appear where the buildings are.
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| Figure 2: The effects of hide boundaries added at building locations. |
