Every road package starts from one line: the centerline. The profile measures elevations along it, cross sections cut across it, quantities follow its stationing. In Civil 3D that line is the Alignment object.
This article shows Civil 3D alignment design the quickest way for anyone who already knows AutoCAD: draw the centerline as a polyline, then let Civil 3D convert it, insert curves and label the stations. Every step has a Civil 3D 2027 screenshot. The data matches the terrain surface from the contour lines article, so you can continue straight on.
What is an alignment in Civil 3D
An alignment is a centerline with stationing. Unlike a polyline, it knows its start, its end, its direction, and the station of every point along it.
An alignment is built from three kinds of element: tangents, horizontal curves (circular arcs) and spirals. Key points carry English names:
| Code | Full name | Meaning |
|---|---|---|
| BP | Beginning Point | Start of the alignment, station 0+000 |
| PI | Point of Intersection | Where two tangents, extended, meet |
| PC | Point of Curvature | Start of a circular curve |
| PT | Point of Tangency | End of the curve, back on a tangent |
| EP | Ending Point | End of the alignment |
Stations are written km+m. 0+133.81 means 133.81 m from the start, 1+250.00 means 1,250 m.
What makes an alignment valuable is the dynamic link. Profiles, cross sections and corridors all hang off it. Change a curve radius and everything downstream updates, as explained in What is Civil 3D.
Ways to create an alignment in Civil 3D
The commands sit on the Home tab, Create Design panel, Alignment button.
| No. | Item | Meaning |
|---|---|---|
| 1 | Alignment button | Opens the alignment creation menu |
| 2 | Create Alignment from Objects | Converts existing polylines, lines and arcs into an alignment |
The three most used commands:
- Alignment Creation Tools: draw from scratch by picking PIs, with curves inserted automatically. Best for a new design on the plan.
- Create Alignment from Objects: convert an existing polyline. Best when the centerline already exists in AutoCAD or comes from another team.
- Create Best Fit Alignment: fit an alignment through a set of points, used when surveying an existing road.
This article uses the second method because beginners can do it straight away, then edits the result with the tools of the first. All three are practised in the Institute's online Civil 3D course.
Preparation: terrain surface and centerline polyline
Open a drawing that has a terrain surface. If you have none, follow the contour lines article with its downloadable sample data.
Draw the centerline with PLINE as in AutoCAD. Straight segments between vertices are enough; do not draw the curves. Civil 3D inserts a curve at each vertex.
The sample route passes through the saddle between the two hills to avoid the highest ground, then cuts the flank of the smaller hill near the end. This gives the profile both fill and cut, useful for the next article.
Step 1: Create alignment from objects
Click Create Alignment from Objects. The command line asks for objects; pick the polyline and press Enter.
| No. | Item | Meaning |
|---|---|---|
| 1 | Selected polyline | Dashed means selected |
| 2 | Command line | CREATEALIGNMENTENTITIES asking for lines, arcs or polylines |
Next it asks for the direction. An arrow shows which way stationing increases. Press Enter to accept or type R to reverse. Direction decides where 0+000 is, so check it before moving on.
Step 2: Set the name, labels and curve radius
The Create Alignment from Objects dialog opens.
| No. | Item | Meaning |
|---|---|---|
| 1 | Name | Alignment name, e.g. TUYEN CHINH. It appears on profiles and in every alignment picker later |
| 2 | Type | Centerline for a road centerline. Other types (Offset, Curb Return, Miscellaneous) are for edges and curb returns |
| 3 | Starting station | Station at the start. Keep 0+000.00m, or continue a previous section's stationing |
| 4 | Alignment label set | Label set. Major Minor and Geometry Points labels major and minor stations plus PC and PT points |
| 5 | Conversion options | Add curves between tangents inserts a curve at each vertex with the Default radius. The default is 200 m; here it is set to 60 m |
| 6 | OK | Create the alignment |
A 200 m default radius is too large for a short route. A curve needs room on both adjacent tangents; with tangents only 85–180 m long and a sharp deflection, R = 200 m will not fit.
The Design Criteria tab sets the design speed.
| No. | Item | Meaning |
|---|---|---|
| 1 | Design Criteria tab | The alignment's design criteria |
| 2 | Starting design speed | Design speed, 100 km/h by default |
Use criteria-based design is unticked by default. When ticked, Civil 3D checks minimum radii against the criteria file. The bundled file follows the US AASHTO standard. Checking against local standards needs your own criteria file, so beginners can leave the box unticked.
Step 3: Read the new alignment
Click OK and the polyline becomes a fully labelled alignment.
The sample alignment is 461.01 m long, in five elements:
| Element | Type | From station | To station | Length | Radius |
|---|---|---|---|---|---|
| 1 | Tangent | 0+000.00 | 0+133.81 | 133.81 m | |
| 2 | Curve | 0+133.81 (PC) | 0+165.13 (PT) | 31.32 m | 60 m |
| 3 | Tangent | 0+165.13 | 0+250.82 | 85.70 m | |
| 4 | Curve | 0+250.82 (PC) | 0+277.17 (PT) | 26.34 m | 60 m |
| 5 | Tangent | 0+277.17 | 0+461.01 | 183.84 m |
The original polyline was erased because Erase existing entities was ticked. Untick it in step 2 to keep it for comparison.
Step 4: Edit the alignment with the Geometry Editor
Select the alignment and a contextual Alignment: TUYEN CHINH tab appears.
Click Geometry Editor to open the Alignment Layout Tools toolbar.
| No. | Group | Used for |
|---|---|---|
| 1 | PI group | Draw by PIs, insert a PI, delete a PI, break an element |
| 2 | Tangent and curve group | Add fixed tangents and curves, or free curves between two tangents |
The buttons on the right of the toolbar open a parameter panel: pick an element to see and edit its radius and length.
The quickest edit is a grip. Select the alignment and blue squares appear at the PIs, tangent midpoints and curve midpoints. Drag a PI grip to move the vertex; the curve follows and downstream stationing recalculates.
Common mistakes in Civil 3D alignment design
The curve cannot be inserted. The default radius is too large for the tangents. Lower the Default radius, or untick Add curves between tangents and add curves later in the Geometry Editor.
Station 0+000 is at the wrong end. The polyline was drawn backwards. Delete the alignment, run the command again and type R at the direction prompt.
The polyline has many short segments. A hand-drawn centerline with vertices close together turns every vertex into a curve. Draw the polyline with as few vertices as possible, one per real change of direction.
Labels overlap. The label set marks every 20 m at 1:1000. Change the annotation scale at the right of the status bar, or choose a sparser set such as Major and Minor only.
Frequently asked questions
Polyline or Alignment Creation Tools?
If the centerline already exists, convert the polyline. For a new design on the plan, Alignment Creation Tools is handier because you set PIs and radii while drawing.
Does Civil 3D support spirals?
Yes. Alignment Creation Tools can insert circular curves with Clothoid spirals at each end. The spiral type is shown on the right of the Alignment Layout Tools toolbar.
How do I change the starting station?
Open Alignment Properties, Station Control tab, and change the station at the start. Profiles, sections and labels follow.
Can I build the alignment from survey points of an existing road?
Yes. Create Best Fit Alignment fits tangents and curves through a set of points or a polyline traced over an old road, and reports how far each point sits from the fitted line. It is the usual start for widening or rehabilitating an existing road.
Why do my station labels read differently from the drawings I receive?
Station format is part of the label style. The metric template writes 0+133.81; some offices prefer Km0+133.81 or a different number of decimals. Edit the station format in the label style once and save it in your company template, so every new alignment follows the house rule.
What comes after the alignment?
Sample the existing-ground profile from the surface and design the finished grade, as in Civil 3D profiles.
Related articles
Civil 3D alignment design is the second link in the surface, alignment, profile, section, corridor chain. To work straight through, go from contour lines to Civil 3D profiles. Command details are in the Autodesk Civil 3D help.
To learn full road alignment design with junctions and quantities, the Institute's online Civil 3D course devotes its third session to alignments and profiles.