SnapIntegrator (QGIS 3.x plugin)

SnapIntegrator is a QGIS plugin for finding unmerged road endpoints inside a boundary polygon, based on a chosen attribute field in a road (line) layer. It helps you detect where two road segments meet geometrically but do not share the same attribute value (for example, different street names or different IDs), which often indicates topology or data-cleaning issues.

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Features

• Works in QGIS 3.16+
• Lets you interactively choose:
  • A polygon layer as a boundary (e.g. municipality, tile, project area)
  • A line layer as a road network
  • A field from the road layer (e.g. road_id, name, code)
• Finds line endpoints that:
  • Belong to non-closed line features (no rings)
  • Are shared by exactly two line features
  • Are strictly inside the selected polygon (not on its boundary)
  • Have different values in the selected attribute field between the two line features
• Outputs results to a new in-memory point layer with:
  • id – running ID of the point
  • field – the name of the selected attribute field
  • val1 – value of that field in the first line feature
  • val2 – value of that field in the second line feature

This makes it easy to spot potential unmerged or inconsistent roads.

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How it works

1. The plugin scans the selected road line layer and collects all endpoints of non-closed lines.
2. For each endpoint, it checks which line features share that exact coordinate (rounded to 6 decimal places).
3. Only endpoints shared by exactly two line features are kept.
4. For each such pair:
   • The plugin compares the values of the user-selected attribute field (field_name) on the two features.
   • If the values are equal, the endpoint is ignored.
   • If the values are different, the endpoint is considered a candidate “unmerged” point.
5. Each candidate point is tested to ensure it lies inside the selected polygon:
   • A small negative buffer (tolerance) is used to avoid points lying exactly on the boundary.
6. All valid candidate points are written to a memory point layer, which is added to the current QGIS project.

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Installation

1. Create a folder named SnapIntegrator in your local QGIS plugins directory, for example:

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

2. Copy the following files into the SnapIntegrator folder:

   • __init__.py
   • snap_integrator.py
   • metadata.txt
   • icon.png

3. Restart QGIS.

4. Go to Plugins → Manage and Install Plugins…, find SnapIntegrator, and enable it.

You should now see a toolbar icon and a menu entry under Plugins → Snap Integrator.

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Usage

1. In QGIS, load:
   • A polygon layer (your boundary).
   • A line layer representing your road network.
2. Select exactly one polygon feature in the polygon layer (this will be used as the search boundary).
3. Click the SnapIntegrator toolbar button (or use the Plugins menu).
4. In the dialog:
   • Choose your polygon layer.
   • Choose your line (roads) layer.
   • Choose the attribute field or you have the option not to choose it from the road layer that you want to check (e.g. road_id, name).
5. Click OK.
6. The plugin will:
   • Analyze endpoints inside the selected polygon.
   • Create a new memory layer named SnapIntegrator_Points with the candidate points.

You can then inspect those points, label them with val1 and val2, or use them for further topology checks.

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Configuration & Notes

• Tolerance
  The plugin uses a tolerance of 0.0001 in layer units to shrink the polygon boundary slightly when checking if a point is inside.
  Adjust this in the code if necessary (for projected vs geographic CRS).

• Supported versions
  Developed and tested for QGIS 3.16; it should also work on newer 3.x versions.

• Limitations

  • Only considers endpoints shared by exactly two line features.
  • Does not attempt to merge or edit geometry: it only detects candidate problem locations.
  • Assumes the line layer and polygon layer are in the same CRS (or set up correctly via QGIS).

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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/SnapIntegrator],(https://github.com/Consortis-Geospatial/SnapIntegrator)
• Issues Tracker: [https://github.com/Consortis-Geospatial/SnapIntegrator/issues],(https://github.com/Consortis-Geospatial/SnapIntegrator/issues)

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License

This project is released under the GNU General Public License v3.