Figure 5.8
Surface with just point
Simply adding surface information to a TIN definition isn’t enough. To get beyond the basics,
you need to look at the edits and other types of information that can be part of a surface.
Refining and Editing Surfaces
and modification to the TIN construction that make it much more usable and realistic. Some
of these edits include limiting the input data, tweaking the triangulation, adding in breakline
information, or hiding areas from view. In this section, you look at a number of ways to refine
surfaces to end up with the best possible model from which to build.
Surface Properties
The most basic steps you can perform in making a better model are right in the Surface Properties
dialog. The surface object contains information about the build and edit operations, along with
some values used in surface calculations. These values can be used to tweak your surface to a
semi-acceptable state before more manual operations are needed.
In this exercise, you go through a couple of the basic surface-building controls that are
available. You’ll do them one at a time in order to measure their effects on the final surface
1. Open the Surface Properties.dwg file.
2. Expand the Surfaces branch.
3. Right-click EG and select Surface Properties. The Surface Properties dialog appears.
4. Select the Definition tab. Note the list at the bottom of the dialog.
5. Under the Definition Options at the top of the dialog, expand the Build option.
The Build options of the Definition tab allow you to tweak the way the triangulation occurs.
The basic options are listed here:
Copy Deleted Dependent Objects When you select Yes and an object that is part of the sur-
face definition (such as the polylines you used in your aerial surface, for instance) is deleted,
the information derived from that object is copied into the surface definition. Setting this option
to True in the Aerial Surface properties would let you erase the polylines from the drawing file
while still maintaining the surface information.
Exclude Elevations Less Than Setting this to Yes puts a floor on the surface. Any point that
would be built into the surface, but is lower than the floor, is ignored. In the EG surface, there
are calculated boundary points with zero elevations, causing real problems that can be solved
with this simple click. The floor elevation is controlled by the user.
Exclude Elevations Greater Than The idea is the same as with the preceding option, but a
ceiling value is used.
Use Maximum Triangle Length This setting attempts to limit the number of narrow ‘‘sliver’’
triangles that typically border a site. By not drawing any triangle with a length greater than the
user input value, you can greatly refine the TIN.
Convert Proximity Breaklines to Standard Toggling this to Yes will create breaklines out of
the lines and entities used as proximity breaklines. You’ll look at this more later.
Allow Crossing Breaklines Determines what Civil 3D should do if two breaklines in a
surface definition cross each other. As mentioned, an (x,y) coordinate pair cannot have two z
values, so some decision must be made about crossing breaklines. If you set this to Yes, you
can then select whether to use the elevation from the first or the second breakline or to average
these elevations.
In this next portion of the exercise, you limit the build options in order to create a better model:
1. Set the Exclude Elevations Less Than value to Yes.
2. Set the Value to 200 and click OK to exit the dialog. Elevations less than 200
will be
Figure 5.9
EG surface after
ignoring low elevations

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