Blog GIS & Territories

When you have a layer of points (field surveys, geocoded addresses, incidents, observations, etc.) and a polygon layer (municipalities, sectors, zones), it is often useful to know which zone each point is located in.

This is what a spatial join allows you to do. And with the MMQGIS plugin, you can do it in just a few clicks, without writing a single line of code.

Objective

For each point, add descriptive information from the polygon in which it is located: name of the municipality, sector, INSEE code, etc.

Required data

• A point layer (Shapefile, GeoPackage, or geocoded CSV format).
• A polygon layer containing the attribute to be reported (e.g., municipality_name, zone_code, etc.).

Both layers must be in the same coordinate system (ideally EPSG:4326 or EPSG:2154 for France).

Steps in QGIS

• Open both layers in QGIS (points + polygons).
• Menu:

  • MMQGIS → Combine → Spatial Join

1. In the dialog box:

   • Target Layer = point layer (the one that will receive the attribute).
   • Join Layer = polygon layer (the one that contains the attributes to be added).
   • Field Operation = First (“Take attributes of first matching feature”).
   • Select the fields to retrieve (e.g., municipality_name).
   • Specify an output file (shapefile or other).

2. Click OK.

Result

A new enriched point layer appears. Each point now contains one or more additional fields from the polygon in which it is located.

Best practices

• If a point does not fall within any polygon, the added fields will remain empty. Remember to check for “out of zone” cases.
• You can choose to add the first intersecting polygon or all of them (but MMQGIS only handles one match per point).
• For complex cases (points in multiple areas, overlaps), the QGIS Processing Toolbox offers more options (including “Join attributes by location (summary)”).

Concrete example

You have:

• schools (points),

• municipalities (polygons).

After spatial joining, each school carries the attribute fields of the municipalities, which is useful for:

• grouping data,
• creating thematic maps,
• generating reports by municipality.

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Atilio Francois