Road Network Curve Smoothing (QGIS Plugin)

Overview

Road Network Curve Smoothing is a QGIS plugin that detects curve points (bends) on a road network within a selected polygon boundary, based on a user-defined angle range. It creates an output temporary point layer of detected curve locations with their corresponding angles, which you can then inspect, symbolize, or use in further analysis.

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Features

• Select one polygon as a boundary (e.g. study area, neighborhood, district).
• Select a line layer representing the road network.
• Specify a minimum and maximum angle (in degrees) to control which curves are detected.
• Automatically creates a memory point layer with:
  • id – sequential ID of each detected curve
  • angle – calculated angle (in degrees) at that curve
• Works with both single-part and multi-part line geometries.
• Ignores vertices outside the selected polygon (uses a small inward buffer to avoid boundary issues).

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How It Works

1. You select exactly one polygon feature from the chosen polygon layer.
2. The plugin builds a slightly shrunk inner boundary from that polygon using a small negative buffer.
3. For each road segment (line geometry):
   • It reads the vertices in order.
   • For every triplet of points (p1–p2–p3), it calculates the angle at p2.
4. Angles that fall within your specified [Min, Max] range and lie within the inner boundary polygon are flagged as curve points.
5. All detected curve points are exported as a temporary point layer and added to the current QGIS project.

Note: The angle is calculated using basic vector math (dot product and magnitudes) and converted to degrees (0°–180°).

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Requirements

• QGIS (3.x)
• PyQt5 / QGIS Python environment (default in QGIS)
• The plugin uses:
  • qgis.core
  • qgis.PyQt
  • qgis.utils.iface

No external Python packages are required beyond QGIS’ own libraries.

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Installation

1. Clone or download this repository.

git clone https://github.com/Consortis-Geospatial/Road_network_curve_smoothing.git 

2. Copy the plugin folder (the folder containing __init__.py, the main plugin file, metadata.txt, curve.png, etc.) into your QGIS plugin directory:

   • Windows:
     C:\Users\<YourUser>\AppData\Roaming\QGIS\QGIS3\profiles\default\python\plugins\
   • Linux:
     ~/.local/share/QGIS/QGIS3/profiles/default/python/plugins/
   • macOS:
     ~/Library/Application Support/QGIS/QGIS3/profiles/default/python/plugins/

   (Adjust the profile name if you use a non-default profile.)

3. Restart QGIS.

4. Go to Plugins → Manage and Install Plugins…

5. In the Installed tab, find Road Network Curve Smoothing and check it to enable.

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Usage

1. Prepare Your Data

   • Add your polygon layer (e.g. area boundary).
   • Add your road network line layer.

2. Select a Polygon

   • Use the Select Features tool to select exactly one polygon in the polygon layer.
   • The plugin requires a single selected polygon as the analysis boundary.

3. Run the Plugin

   • In the QGIS menu, go to
     Plugins → Road Network Curve Smoothing
     or click the toolbar icon if visible.
   • The plugin dialog will appear.

4. Set Parameters

   • Polygon Layer (Boundary): choose your polygon layer.
   • Line Layer (Roads): choose your road network layer.
   • Angle Range (degrees):
     • Min: default = 45°
     • Max: default = 75°
   • Click Run.

5. Results

   • If suitable curve points are found:
     • A new memory layer named Detected_Curves is added to your Layers panel.
     • It contains a point at each detected curve with:
       • id: unique integer
       • angle: angle in degrees (rounded to 2 decimal places)
   • If no curves are found within the specified range, an info message appears in the QGIS message bar.

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Dialog Description

The main dialog includes:

• Polygon Layer (Boundary)
  Drop-down to choose the polygon layer used as boundary.

• Line Layer (Roads)
  Drop-down to choose the road network line layer.

• Angle Range (degrees)

  • Min and Max values using double spin boxes from 0 to 180°.
  • Defaults to 45°–75°.

• Run Button
  Starts the processing using the selected layers and angle range.

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Messages & Error Handling

The plugin uses the QGIS message bar (iface.messageBar()) to show feedback:

• Warning:
  • If polygon or line layer is not selected.
  • If the number of selected polygons is not exactly one.
• Info:
  • If no curve points were found in the given angle range.
• Success:
  • Shows how many curve points were exported and the angle range used.

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Limitations & Notes

• The plugin currently supports:
  • Polygon boundary layers.
  • Line layers (roads) with polyline geometries (single or multi-part).
• The inner boundary uses a fixed small negative buffer (tolerance = 0.0001).
  If your data uses very large or very small coordinate values, you may want to adjust this value in the code.
• The output is a memory layer; if you want to keep it permanently, right-click it and choose:
  • Export → Save Features As…

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Code Structure

• RoadNetworkCurveSmoothingDialog

  • Builds the user interface (layer selections, angle inputs, run button).

• RoadNetworkCurveSmoothing

  • Handles plugin initialization (initGui, unload).
  • Shows the dialog (run).
  • Processes the selected layers and generates the output layer (process).
  • Calculates angles between three points (calculate_angle).

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License

This plugin is released under the GPL-3.0 license.

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Support and Contribution

• Homepage: [https://github.com/Consortis-Geospatial],(https://github.com/Consortis-Geospatial)
• Author: Dimitra Pappa -Consortis Geospatial
• email: pappa@consortis.gr
• Repository: [https://github.com/Consortis-Geospatial/Road_network_curve_smoothing],(https://github.com/Consortis-Geospatial/Road_network_curve_smoothing)
• Issues Tracker: [https://github.com/Consortis-Geospatial/Road_network_curve_smoothing],(https://github.com/Consortis-Geospatial/Road_network_curve_smoothing)