Creating Curved Polygon Tunnel Shapes to Set Degrees in QGIS
In Geographic Information Systems (GIS), modeling a "tunnel" shape—a curved polygon with a consistent width—is a common task for urban planners and civil engineers. While the standard polygon tool is linear, QGIS provides several Advanced Digitizing methods to create perfectly curved geometries with specific degree offsets.
1. Method 1: The 'Advanced Digitizing' CAD Tools
For the most precise control over degrees and distances, the Advanced Digitizing Panel (CAD Tools) is your best resource. This allows you to lock angles and lengths as you draw.
- Enable CAD Tools: Right-click any toolbar and check Advanced Digitizing Panel.
- Set the Pivot: Click your starting point. Press
Ato lock the angle (degrees) andDto lock the distance. - Drawing Curves: Use the Circular String tool instead of the standard "Add Polygon" tool. This allows you to create true curves based on three points or a radius.
2. Method 2: Creating a Tunnel via Buffered Arcs
If your "tunnel" needs to follow a specific centerline, the most accurate way to ensure a uniform width is to create a curved line first and then Buffer it.
- Draw the Centerline: Use the Shape Digitizing Toolbar to create a "Circular String." This ensures your curve is mathematically sound.
- Calculate Degrees: If you need a specific arc (e.g., a 90-degree turn), use the Numerical Digitize plugin to input coordinates or the CadDigitize plugin to set the sweep angle.
- Apply Buffer: Go to
Vector > Geoprocessing Tools > Buffer. Set your distance (half the tunnel width) and increase the Segments count to at least 30 to ensure the curve looks smooth rather than jagged. - Cap Styles: Set the "End cap style" to Flat if you want the tunnel ends to be perpendicular to the path.
3. Comparison of Digitizing Tools for Tunnels
| Tool Name | Best For... | Curve Type |
|---|---|---|
| Circular String | True mathematical arcs. | Non-segmented curves. |
| Vect-Tools (Plugin) | Offsets and parallel lines. | Exact parallel buffers. |
| QAD (Plugin) | AutoCAD-like commands. | Complex geometric tunnels. |
4. Advanced: Using Geometry Generator for Dynamic Tunnels
If you want to visualize a tunnel without creating a new file, use the Geometry Generator in the Layer Styling panel. This is perfect for "what-if" scenarios.
Use the following expression to turn a curved line into a 10-meter wide tunnel on the fly:
buffer( $geometry, 5, segments:=20, cap:='flat' )
5. Exporting for Engineering (CAD)
If your curved polygon needs to move into AutoCAD or Civil 3D, ensure you export using the DXF format. In the export settings, check "Force 2D output" and ensure your Coordinate Reference System (CRS) is set to a Projected CRS (like UTM) rather than WGS84, otherwise, your degree-based measurements will be distorted.
Conclusion
Creating a curved polygon tunnel to set degrees in QGIS is achieved by combining Circular String digitizing with Buffer geoprocessing. By utilizing the Advanced Digitizing panel in 2026, GIS professionals can maintain high spatial accuracy while modeling complex infrastructure. Remember: always check your "Segments" count to ensure your curves remain smooth across all zoom levels.
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