CAD Clinic: Getting Started with Civil 3D Alignments

8 May, 2006 By: Mike Choquette

Civil 3D roads, railways, runways, bike paths and other linear designs all begin with an alignment object.

Alignments are used in nearly every civil engineering project to help layout roads, railways, runways, walking and bike trails -- in fact, any kind of linear design feature. In these types of applications, alignments represent centerlines, lane boundaries, shoulders, right-of ways and similar features. In addition, many other projects can benefit from alignments such as swales, waterways, utilities and some types of earthwork such as levee, dam and landfill designs.

Autodesk Civil 3D alignment objects can include line segments, circular arcs and spirals. Vertical data along an alignment is stored in a profile, as discussed in a previous CAD Clinic column. In Civil 3D, alignments are combined with profiles and assemblies (proposed cross sections) to create 3D design models called corridors. Alignments in Civil 3D are dynamic objects linked to the profiles and corridors that depend on them, which means that changing an alignment through a table of design data or through on-screen grip editing automatically redefines the alignment and updates profiles and corridors accordingly. This ability is a dramatic and long-awaited step beyond the functionality provided by Land Desktop.

Object Styles
Civil 3D uses object styles to control the display and annotation of objects such as points, alignments, profiles and so on. Alignment geometry and labels automatically appear as you create alignments in real time based on selected alignment styles and alignment label styles. Generally speaking, most firms customize these styles to match their company standards or client deliverables.

If you're just getting started with Civil 3D, begin your drawings with the _AUTODESK CIVIL 3D IMPERIAL BY LAYER.DWT template (Autodesk Civil 3D (Imperial) NCS Classic.dwt in Civil 3D 2007), which contains a large number of sample styles. These sample styles can serve as examples of what Civil 3D is capable of, and could be useful as the start point of your own Civil 3D Standards. If you have an existing company template, start a new drawing with the same template and import your template contents using the AutoCAD Insert command. This process first requires you to make a copy of your template with the .dwt file extension changed to .dwg.

Importing Alignments
You can import alignments from other applications or build them directly inside Civil 3D. Point Groups, Surfaces, Alignments , Profiles, Parcels, Pipe Systems and some Corridor elements can be imported and exported easily with the LandXML format. In Civil 3D 2007, the import/export commands are available under the File menu, but in earlier releases they were under the General pull-down menu. Users also can import data directly from Land Desktop project files from commands in these same menus.

Keep in mind that changes made to data imported from Land Desktop projects are not automatically applied to the original Land Desktop project. However, if you want to update a Land Desktop project database to include work created in Civil 3D, you can do so by opening a Civil 3D 2006 drawing in Land Desktop 2006 and choosing the Projects/Extract Civil 3D Data command.

Creating Alignments
You can create new alignments directly within Civil 3D from polylines or create them interactively with the Alignment Layout tools. If the geometry for your alignment already exists in your drawing, such as for an existing highway baseline that may be provided by a surveyor as lines and arcs, first join that geometry into a single polyline. You can create an alignment from your joined polyline with Alignments/Create Alignment from Polyline. This approach may be best when you need to perform some AutoCAD acrobatics to determine the first draft of your alignment, such as a subdivision road constrained by multiple property line offsets.

When creating an alignment from a polyline, several options are available in the Create Alignment dialog (figure 1). Alignments, profiles, cross sections, grading groups and parcels are stored in sites to allow designers to organize them appropriately. A site could represent a design alternative, a phase of a project, a separate geographic location or any other logical breakdown. You can choose an existing site from the drop-down list or create a new site on the fly with the button to the right of the drop-down list (figure 2).

Figure 1. Create an alignment from polyline options.

Figure 2. Creating a new site on the fly within the Create Alignment dialog.

The Name and Description fields allow you to identify the alignment as you see fit. The Starting Station field is where you identify the station at the start point of the polyline. Note that this station is easy to modify later if you would rather define the starting station somewhere else. The Alignment Style field lets you choose the style that controls the display of the alignment geometry itself -- specifically the alignment tangents, curves, spirals, extension lines, points of intersection, pass-through points, station reference point and direction arrows. You can choose to adjust or disable the display of all these elements through the Alignment Style. The Alignment label set option controls the alignment labels such as station text, station point labels and related annotation. The default layer the alignment is placed on is the layer specified in the Edit Drawing Settings Command. To change this setting, right-click on drawing name in the Settings tab of the Civil 3D Toolspace and choose Edit Drawing Settings. The conversion options let you add curves automatically to angle points in the alignment (points of intersection without curves) and automatically delete the original polyline, if you wish.

The alignment direction initially matches the direction of the original polyline. If you would prefer the alignment to run in reverse of the original polyline, use the Alignments/Reverse Polyline command.

Coming Soon
In a coming article, I'll discuss another way to create new alignments using Create by Layout and also discuss the alignment editing tools.

Civil 3D alignment objects offer users a dynamic, easily updatable foundation for road design and similar linear design tasks. This long awaited toolset is an enormous step forward over the functionality of Land Desktop and similar software packages.

About the Author: Mike Choquette

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