Connecting Objects to Objects on AutoCAD Civil 3D

The most important type of relationship that you’ll see in this chapter is the one between objects. A typical land development project is a collection of dozens of mini-designs that often tie in to one another. For example, a road is designed first by drawing the 2D path of its centerline, then the proposed changes in elevation along that centerline, and finally the lanes, curbs, and sidewalks extending outward from that centerline. To provide drainage during a rainstorm, ditches must be installed along the sides of the road. The location and depth of these ditches can be traced back through the design process the entire way to the layout of the road centerline. If the layout of the centerline needs to change for some reason, that change must propagate downstream through the design process, ultimately changing the location and depth of one or more ditches.

In Civil 3D, these connections between elements of the design are present regardless of the tool that is used. Before Civil 3D, these connections had to be managed manually by engineers and designers, and every aspect affected by a design change had to be fixed manually. With Civil 3D, these connections can be built into the design by establishing relationships between the road centerline, the roadside ditches, and everything in between.

Quick Tips

<Osnap Off>
will prevent your cursor from locking on to objects in the drawing that are near to it.

To see how object relationships are leveraged to make design changes in a drawing, follow these steps:

1. Open Object Relationships.dwg located in the Chapter 02 class data folder.

2. Press the F3 key, and observe the command line. If it reports <Osnap On>, then press F3 again. If it reports <Osnap Off>, this is the correct condition needed for this exercise and you can move on to the next step.

Quick Tips

Be sure that your command line is docked at the bottom of your screen and that the background color is set to white before proceeding with these steps.

3. Click the top-right viewport, which shows a profile of the road design. The black lines represent the elevations along the centerline of the new road. The blue lines represent storm drains and pipes connecting them.

4. Click the black line representing the road profile. Zoom in until you can clearly see the triangular grip located at the intersection of two lines.

5. Click the triangular grip, and drag it upward to a location just below the top edge of the profile view grid, as shown in figure. Watch the profile view and 3D view carefully. Notice how the 3D view of the road updates in the bottom-right view, including the height of the drain labeled Inlet 2. In the profile view (top right), the top of the drain is elevated to match the change in the road elevation.

Grip editing the profile.

This simple exercise illustrates the power of relationships between objects. The ease with which you just updated the design may cause you to take the underlying processes for granted; however, there is a lot happening behind the scenes. The following is a general account of the events that took place when you changed the location of the triangular grip:

• The slopes of the lines leading into that triangular grip were changed to match the new location of the grip.
• The parabolic curve geometry at the location of the grip was updated automatically.
• The corridor object, which represents a 3D model of the road, was automatically rebuilt and updated to match the new profile geometry.
• A surface representing the pavement, concrete, and earthen embankment elevations of the corridor was automatically rebuilt.
• The storm drain updated its top elevation to match the surface in the previous step.
• The 3D representation of the storm drain was automatically updated (bottom-right view).
• The profile view representation of the storm drain was automatically updated (top-right view).

A simple grip edit triggered a chain of events that might have taken an hour or more to update manually. In addition to all of this, there were other changes that took place that did not affect the design of the storm drain. This is the power of the Civil 3D dynamic environment. You should know, however, that the existence of these relationships is not necessarily automatic. They have to be considered and at times consciously built into the design by the Civil 3D user.