In June's CAD Clinic we 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. This article begins by describing the "freedom" constraint of alignment elements, which controls how they interact with neighboring elements in the alignment. I'll also discuss one technique for creating new alignments using the Civil 3D alignment layout tools.
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. Each element (a line, circular curve or spiral segment) is defined as fixed, floating or free. You cannot change this constraint directly, but you can delete them and recreate alignment elements with a different constraint type.
Fixed elements are the least dynamic and are intended for elements that are not necessarily dependent on their neighbors. For example, if you accidentally change a fixed curve's radius, the curve length would change, but adjacent fixed elements would remain the same. This would cause the curve to lose tangency with its neighbors, but you may prefer that outcome if the alignment was not meant to be adjusted at all (like an existing highway baseline that you aren't supposed to edit).
Free and floating elements often are more useful for alignments that are subject to change, however, such as those for proposed roadway centerlines. The geometry of free elements is affected by changes to objects on either side of them. (I'll discuss free elements in a future CAD Clinic article). Floating elements depend only 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) will translate accordingly while holding all of their initial constraints (length, radius, pass-through-points, etc.). For example, the alignment below begins with a fixed line followed by a series of floating curves and a floating line that together define a cul-de-sac (figure 1). Changing the end point of the fixed line causes all the floating elements to translate while maintaining the original length of the floating line, as well as the original length, radius and direction of the curves.
Figure 1. Changing an alignment with floating elements.
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Creating an Alignment from Known Line and Curve Data This approach is helpful if you know the geometric properties (length, radius, etc.) of your alignment elements ahead of time. This situation is often the case when designing a new road on a very constrained site or when recreating an existing baseline from line and curve data. With this approach you sequentially define alignment elements -- lines, circular curves and spirals -- one after another. Downstream elements normally are defined as tangent to the preceding elements, but this isn't required. Floating elements often are useful in an alignment defined with this approach.
In these cases you will want to begin your alignment with the Alignments/Create by Layout command. This command calls up the Alignment Librarian, which was discussed previously in the June article. After you have settled on an appropriate name and settings for your new alignment, press OK and the Alignment Layout toolbar appears (figure 2).
Figure 2. The alignment layout toolbar.
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Start your alignment by choosing an initial element from either the line, curve or spiral drop-down menus. Often you will want to begin with a fixed element followed by floating and other fixed elements (free elements are added between two elements rather than on the end of one). For example, to begin with a fixed line defined by two points, choose the Fixed Line tool from the layout toolbar (figure 3).
Figure 3. Create a fixed line with two known points.
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If you would like to identify your start point based on a northing and easting value, grid coordinate, latitude/longitude, another alignment's station or offset value, etc., use the appropriate tool on the Transparent Commands toolbar (figure 4).
Figure 4. The Transparent Commands toolbar highlighting the Northing and Easting tool.
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You could locate the second point of a two-point fixed line by using one of the above mentioned transparent commands, picking it graphically or calculating it through one of the angle and distance tools also located in the Transparent Commands toolbar. (Angle measurements include bearings, simple angles, azimuths, etc.)
After the initial element is defined the other fixed and floating elements are added in sequence along the alignment. For example, to add a floating curve to the end of another element, choose the desired type from the dropdown list shown below (figure 5).
Figure 5. Creating a floating curve with a specified length and radius that is always tangent to the initial line.
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When adding the curve above you will be prompted to select the end of the preceding line, a direction (clockwise or counterclockwise) and a radius and length. Later you can change the floating curve's radius and length value by editing the alignment properties, but it will always remain tangent to the line element it was initially attached.
Summary
Alignment elements have a freedom constraint that defines 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.
Coming SoonThe next article on alignments will consider the free constraint in more detail, as well as discuss a technique of creating alignments by first specifying PIs (points of intersection) and creating free curves automatically between them. A future article will discuss techniques for editing Civil 3D alignments.
). It is sometimes necessary to delete an entity and replace it with another in order to apply different constraints (such as to replace a free curve with a set radius with a free curve through a designated pass-through point instead). Deleting a free curve causes the alignment to be re-drawn through the PI as an angle point. Deleting other kinds of curves or line segments can cause a gap to form in the alignment that is not automatically reconnected. You can bridge this kind of gap by adding more lines and curves, or by dragging line endpoint grips together to form new PIs. 
