CAD Clinic: Civil 3D – Editing Alignments with Civil 3D

6 Nov, 2006 By: Mike Choquette

Use the editing toolbar to dynamically adjust and refine your alignment

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. The September column examined the free alignment constraint and how to use it in a design. This article completes our review of Civil 3D horizontal alignments by considering techniques to dynamically adjust them once they have been created.

AutoCAD Modify Commands
The simplest way to adjust a Civil 3D alignment is to change it with standard AutoCAD commands like Move, Rotate, Stretch and Grip Edits. A note to former users of Land Desktop: yes, this actually works -- and works well.

Civil 3D automatically updates an alignment definition as soon as you complete one of these commands. The AutoCAD Undo and Redo commands are also a valid choice if you change your mind, so go ahead and explore all those alternatives! If you are new to Civil 3D and AutoCAD in general, we recommend that you also review other CAD Clinic articles about AutoCAD's Modify commands.

Of these, Grip Editing is probably the most impressive to new users. Civil 3D alignments have grips available at the PIs (points of intersection), as well as the center point and endpoint of lines and curves (though some curves have unique grip configurations based on the constraints chosen at creation). If a free circular curve was assigned a certain radius, moving its PI through a click-and-drag would cause the curve and its adjacent tangents to be re-drawn while holding the same radius value.

Civil 3D automatically updates an alignment's definition after applying standard AutoCAD Modify commands, like this grip-edit of the point of intersection.

The grip at the center of a free circular curve can be relocated as a pass-through point, which causes the curve radius and accompanying line segment lengths to change in order for them all to remain tangent. Gripping and moving the center point of a line segment lets you relocate that line while maintaining the same bearing, which causes adjacent curves to update accordingly. I believe most AutoCAD users will find this interaction very intuitive. If you haven't experimented with profiles and corridors yet, keep in mind that changing the alignment can automatically update the existing ground profiles and cross section linework. This flexibility is an excellent example of the dynamic design environment provided by Civil 3D.

Graphical Edits with the Alignments Toolbar
Beyond the core AutoCAD modifiers listed above, Civil 3D also allows users to graphically edit an alignment through features in the Alignments Layout Tools toolbar. This toolbar appears when you launch the Alignments / Edit Alignment Geometry from the pull-down menus, or if you select an alignment object, right-click and choose Edit Alignment Geometry. Note that this toolbar is only visible during alignment creation and editing; it disappears from your screen when not in use. While working with this toolbar you may want to stop yourself from using the Escape key since it will end your alignment editing session and cause the toolbar to disappear. Although this disappearing act conserves screen real estate, it can be a little disconcerting. Instead, when you want to abandon an edit, use the Undo button in the toolbar. (If you're like me, that may take some getting used to!) So, if the toolbar ever evaporates on you before you're done editing, bring it right back in with Alignments / Edit Alignment Geometry.

The Alignment Layout Tools toolbar with the PI commands circled.

On this toolbar, the graphical edit commands include Insert PI, Delete PI and Break Apart PI (circled above). Creating a new PI inserts an angle point at the selected location, which can later have a curve fit through it with one of the Create Curve Commands (covered in the July and September articles). Deleting a PI does just that -- it deletes a point of intersection and any curves that pass through it. This approach re-draws an alignment from the first tangent's start point through to the following PI if free curves are present (illustrated below). Adding or deleting a PI is a significant alignment change that may only be necessary when abandoning one alternative for another, very different layout.

The same alignment before (left) and after (right) the circled PI was deleted.

The Break Apart PI command removes the single PI grip and replaces it with two line segment endpoint grips. The advantage to having a broken-apart PI is that you can select one of these grips and adjust it independently of the other, which allows you to change one tangent's bearing or length without adjusting the bearing of the other.

A "broken apart" PI.

If you would like to delete a curve or line segment, use the Delete Sub-Entity command ( Delete Sub-Entity command ). 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.

Civil 3D's tabular editing tools (from left to right): Pick Sub-entity, Sub-entity Editor and Alignment Grid View.

Tabular Edits with the Alignments Toolbar
There are also a handful of commands that allow you to adjust alignment features in one of two tabular views. The Sub-entity Editor is a modeless dialog box (titled Alignment Layout Parameters) that lists design properties of an alignment line or curve. Those features that can be edited numerically appear in bold, such as the curve radius shown below. To activate the editor click on the Pick Sub-entity button and choose an entity to review. To dismiss the editor, click on the Sub-entity Editor button on the toolbar or click the red X in the upper right of the dialog box.

Changing an alignment's curve radius through the Sub-entity Editor.

The Alignment Grid View command calls up the Alignment Entities Vista in the Panorama window, where you can review each alignment entity as a row in a table. As with the Sub-entity Editor, design features that can be edited numerically appear in bold.

Editing alignment features through the Panorama window.

As a final note, remember that there are Undo and Redo command buttons located in the far right of the toolbar. These let you revisit previous changes while still in the Edit Alignments command. Keep in mind that the core AutoCAD Undo command (Edit / Undo) undoes all edits completed during your previous edit alignment command, not just the last individual change you made. Therefore it's preferable to use Undo and Redo from the toolbar whenever possible, since they only reverse the last incremental change made during an alignment editing session, not the entire session.

As the last portion of our review of Civil 3D alignments, this article overviews the tools available to adjust and refine horizontal alignments. These dynamic tools offer unprecedented advances over similar editing steps provided by Land Desktop and other traditional civil engineering packages.

About the Author: Mike Choquette

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