Understanding Data Settings for Surfaces in AutoCAD Civil 3D

Various types of data can be added to AutoCAD Civil 3D surfaces to ultimately create the contours that we seek to display. In the case of Boundaries, Breaklines, and Contours, there are settings presented that are often ignored because they are not understood. The purpose of this blog is to discuss those settings.

Boundaries

Boundaries are added to omit displaying areas of the surface. We can choose to omit the display of the surface outside a boundary by designating it an outer boundary. Hide boundaries prevent the display of information inside of its limits; we would apply hide boundaries to a building outline to “hide” the contours inside the building outline. Show boundaries display areas inside hide boundaries, as with the case of courtyards enclosed by a building. These are examples of different boundary types.

Breaklines

Breaklines represent linear features of a site such as edges of pavements, sidewalks, shoulders, curbs, or top of banks. They also represent crowns and flowlines. Adding breaklines to a surface causes a change to the triangulation. TIN lines are reconfigured to be drawn along the breakline from vertex to vertex. The program will interpolate along the breakline for contour elevations. When a contour is approaching the breakline, it will seek out its elevation along that breakline before being drawn beyond it. TIN lines will be drawn along the contours to maintain control over the shape of the entities representing the contours.

Contours

When contours or polylines representing contours are added to a surface, they will trigger a breakline behavior. TIN lines are drawn along the polylines forcing a continuous contour line along those polylines.

Non-Destructive or Not?

In the Add Boundaries dialog, we would choose the type of boundary we are adding. By default, the Non-destructive breakline checkbox is filled, but what does this mean?
In the image below, we see a surface displaying TIN lines. The red polyline rectangle represents an outer boundary that has yet to be added to the surface definition.

After adding the rectangle as an outer boundary with the non-destructive breakline option selected, we notice that the TIN lines outside the boundary are no longer displayed. TIN lines intersecting the boundary have been trimmed at the boundary limits thereby not “destroying” the TIN lines.

If the rectangle is added as an outer boundary with the non-destructive breakline option unchecked, we notice that the TIN lines outside the boundary are no longer displayed. In addition, the TIN lines crossing the boundary have been deleted thereby “destroying” those TIN lines.

Mid-ordinate Distance

This setting is important when we have arcs in our surface definition. The Mid-ordinate Distance setting is present when adding boundaries, breaklines, and contours. This setting causes AutoCAD Civil 3D to place supplementary TIN points along the arc.

The result of adding data to a surface is a TIN (triangulated irregular network). Triangles have flat edges, not curved. Without the mid-ordinate distance setting, the program would draw a TIN line from one endpoint of the arc to the other, losing the shape of the arc in the surface definition. The distance value entered defines the perpendicular distance between the midpoint of an arc and its chord. AutoCAD Civil 3D will create these chords (TIN lines) along the arc maintaining the mid-ordinate distance setting, thus maintaining a close resemblance of the shape of the arc in the TIN.

Weeding Factors

The settings for Weeding Factors can be found when adding breaklines or contour data.

In the Add Breaklines dialog and the Add Contours dialog, you can set weeding factors to omit extraneous vertices. The distance factor tells AutoCAD Civil 3D to look at three adjacent vertices. If the distance between the first point and the third point are less than the distance specified in the dialog, a judgement is made based on the angle specified. If the three points were connected by segments, and if the deflection angle between the two segments was less than the specified value in the dialog, then the middle vertex is omitted. The image below can be found in the AutoCAD Civil 3D Help file.

Supplementing Factors

Supplementing factors will add vertices to a boundary or contour segment. This helps to distribute the triangles uniformly along those components.

The distance specified in the Add Breakline and Add Contour Data dialogs would be the target increment at which the vertices will be placed. The image below can be found in the AutoCAD Civil 3D Help file.

The mid-ordinate distance value in Add Breakline and Add Contour Data dialogs controls how segments are created around curves, as described in the above section entitled Mid-ordinate Distance.

It’s very important that we understand the settings affecting the creation and appearance of our surfaces. Doing so will lead to more precise cut and fill analysis. Creating an accurate surface will help us to determine proper depths for our pipes. Being able to visualize a well-made existing grade in a profile view allows us to determine optimal design grades for our roads. All in all, the surface is the most fundamental element in your site model.