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# CAD Clinic: Civil 3D -- Creating Complex Alignments with Civil 3D, Part 2

6 Sep, 2006

### Use the constraints of alignment objects to control the interaction with neighboring elements.

In May's CAD Clinic I covered the basics of what a Civil 3D alignment object is, as well as how to import and convert polylines into alignments in Civil 3D 2007. In July's column I described the usefulness of fixed and floating alignment constraints when laying out an alignment with known line and curve data. This article will examine the free alignment constraint and how to use it in a design.

As a brief review, Civil 3D alignment objects are used to define construction baselines for roads, railways, runways, bike paths and other linear design projects. Alignments also can serve in a variety of supporting roles such as controlling the edges of variable-width roadways, defining paths of utility networks, laying out meandering sidewalks or determining the offsets to irregular right-of-ways.

Fixed, Floating and Free Alignment Elements
An alignment can contain a combination of fixed, floating and free elements (lines, circular curves or spiral segments). Fixed elements are the least dynamic. They are intended for elements whose properties remain as static as possible when their neighbors are adjusted -- even so much as to lose tangency. Floating elements are only dependant on the object before them in the alignment. If a preceding object is moved, stretched or otherwise adjusted, a floating element (and everything following it) translates accordingly while holding all of their initial constraints (length, radius, pass-through-points, etc.) Free elements are almost entirely dependant on the objects immediately before and after them in an alignment.

Free lines are drawn between a fixed or floating curve (or spiral) on either side, and are automatically located tangent to both. If either neighbor changes, the line is automatically relocated so that it remains perpendicular. (A free line's length and bearing are completely dependant on the geometry of the adjacent elements.)

 Figure 1. A free line connecting two fixed curves is automatically redrawn after one of the adjacent curves is adjusted.

You can draw free curves between lines or spirals with a given radius or pass-through point. Similar to free lines, if a free curve's neighbors change, then the curve is completely redrawn to remain tangent on both sides, maintaining either the original radius or pass-through point.

You can include free spirals as single spiral elements, a pair of compound or reverse spirals, 3-element spiral-curve-spiral combinations, as well as compound or reverse spiral-line-spiral combinations. Several of these options are new to Civil 3D 2007.

You can add free elements when an alignment is created or edited through these drop-downs in the Alignment Layout toolbar:

 Figure 2. Location of the free compound Spiral-Line-Spiral command where you can specify the length of the central line element.

Create an Alignment by Locating Points-of-Intersection
When a project allows for some flexibility in the alignment layout, a great way to create alignments is to locate the first draft of your tangents (line segments) and allow Civil 3D to add free curves automatically in-between. With this approach you can edit the tangents or drag the PIs (points-of-intersection) and the curves will update appropriately.

To make a new alignment, choose Alignments / Create by Layout, which calls up the Alignment Layout toolbar. The first step in this approach is to set a default curve radius and spiral settings. Once drawn, free curves assigned a specific radius can have their radius adjusted at any time through editing techniques (discussed in a future article). If spiral transitions are not desired, leave the Spiral In and Out boxes unchecked, as shown below.

 Figure 3. The Curve and Spiral Settings dialog box.

Next, choose to layout the first draft of your alignment elements through the Tangent-Tangent (With Curves) command.

 Figure 4. The command to create alignment tangents by locating points-of-intersection, with free curves automatically added in-between.

Civil 3D now prompts you to identify points-of-intersection where your alignment tangents meet. This command automatically adds fillet curves through the PIs as you go based on the default options in the Curve and Spiral Settings dialog box. Once your first draft is complete, you are free to adjust the tangents or PI locations, knowing that the free curves update automatically in order to remain tangent.

 Figure 5. As this PI is adjusted, the adjacent line segments and free curves are automatically updated (where the free curves have maintained the same radius value).

Summary
Alignment elements have freedom constraints that define how individual lines, arcs and spirals are related to their neighbors. These relationships govern how graphical updates are propagated through an alignment and allow for different techniques of building sophisticated, rule-based alignments. Free entities automatically adjust to remain tangent when neighboring entities are modified.

Coming Soon
November's CAD Clinic article, the last focusing on an initial review of alignments, will consider techniques for editing Civil 3D alignments.

# About the Author: Mike Choquette

 AutoCAD Tips! Autodesk Technical Evangelist Lynn Allen guides you through a different AutoCAD feature in every edition of her popular "Circles and Lines" tutorial series. For even more AutoCAD how-to, check out Lynn's quick tips in the Cadalyst Video Gallery. Subscribe to Cadalyst's Tips & Tricks Tuesdays free e-newsletter and we'll notify you every time a new video tip is available. All exclusively from Cadalyst! Follow Lynn on Twitter

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