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Alignment Tangency Constraints in Civil 3D ensure smooth transitions between alignment segments by enforcing tangency conditions. Tangency constraints are essential for maintaining design standards in roadways, railways, or other linear infrastructure projects. Here's how to work with alignment tangency constraints in Civil 3D:

Understanding Tangency in Alignments
Tangency Constraints ensure that curves, tangents, and spirals connect seamlessly without sudden changes in direction.
Civil 3D offers tools to check, enforce, and edit tangency during or after alignment creation.
1. Create an Alignment with Tangency
A. Using the Alignment Layout Tools
Open Alignment Creation:
Go to the Home Tab Create Design Panel Alignment Create Alignment from Layout.
Use the Alignment Layout Toolbar:
After creating a new alignment, the Alignment Layout Tools toolbar appears.
Use the tools for Line, Curve, and Spiral creation to add tangents and curves.
B. Automatic Tangency
When drawing an alignment, Civil 3D attempts to create tangency automatically:
Line to Curve: Adding a curve between two tangents ensures tangency.
Curve to Spiral: Adding spirals between curves ensures smooth transitions.
PI-Based Tools: Tools like Free Curve (Between Two Entities) or Free Spiral-Curve-Spiral automatically enforce tangency.
2. Check for Tangency Issues
Alignment Grid View:
Select the alignment and click Geometry Editor from the contextual ribbon.
In the Alignment Layout Tools, open the Grid View to check geometry data for tangency issues.
View Curve Data:
Look for abrupt changes in curvature or transitions between segments that aren't tangent.
3. Fix Tangency Issues
A. Using the Grid View
Open Alignment Properties:
Select the alignment, right-click, and choose Alignment Properties.
Modify the Geometry:
In the Grid View, edit values like radius, length, or angle to enforce tangency.
B. Edit with Grips
Select the alignment in the drawing.
Use Grips to adjust the points of intersection (PI) or curve radii.
Ensure the alignment dynamically adjusts to maintain tangency.
C. Recreate Curves or Transitions
Remove Non-Tangent Segments:
Delete problematic segments using the Delete Sub-entity tool.
Redraw with Tangency Tools:
Use Free Curve (Between Two Entities) or Free Spiral-Curve-Spiral to redraw with enforced tangency.
4. Enforce Tangency Constraints
A. Alignment Layout Tools
Tangency Option:
In the Alignment Layout Tools, enable Maintain Tangency (checkbox) while adding or editing geometry.
Dynamic Updates:
Civil 3D will automatically adjust the alignment to maintain tangency as you modify it.
B. Add Spirals Between Tangents and Curves
Use Spiral Tools:
Add Spiral-In or Spiral-Out transitions to smooth transitions between tangents and curves.
Adjust Spiral Parameters:
Specify parameters like length or radius to meet design criteria.
5. Verify Alignment Tangency
Check Alignment Properties: