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To formulate my question with a concrete example: I have two multi-polygon layers of the same city.

  • Layer A has polygons of the city divided in parts e.g. north, south, west, east.
  • Layer B has smaller polygons that represent green e.g. trees, forests, parks and such.

Now I want to add a new column "city_part" in the attribute table of layer B. And likewise I'd want an either QGIS-expression- or Python-coded script, that can figure out in which polygon of layer A every single polygon of layer B lies. So the concrete result would be a layer B in which every polygon's got a new attribute, telling whether it resides inside north, south, west or east. How can this be done?

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    a spatial join would also work. use the tool join attributes by location.
    – csk
    Dec 20 '19 at 21:19
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One way of many:

  • install refFunctions plugin
  • with your greens layer active

    • open Field Calculator
    • Create a new field

      • choose Output field name (e.g. 'city_part')

      • choose Output field type to match that of the city parts layers id column (ì.e. Text)

    • use expression:

      geomintersects( '<city_parts_layer_name>', '<city_parts_id_column_name>')
      

      and hit Ok

This assumes the city parts polygon id has the values north, south, east, west. It also assumes uniqueness of the intersections; if one of the smaller polygons intersects multiple city parts, it is internal order based which one will get assigned.

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  • geomintersects() wasn't recognized, which plugin do I have to install to have this function? Dec 20 '19 at 11:44
  • Ops, forgot: those functions are part of the refFunctions plugin; see update for link
    – geozelot
    Dec 20 '19 at 11:49

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