[GIS] Use field calculator in QGIS so get number of points in polygone and store it in attribute table

field-calculatorpoint-in-polygonqgis

I have to layers, one point one polygon (with 6 areas) stored as shapefiles.
Now I want to count the number of points in each area and write it into the attribute table of the polygons. A column sum_poi already exists.

Can I use the field calculator to populate the column in the polygon layer?

Thanks for your help.

Best Answer

It is not that easy. You have to use Vector -> Analysis Tools -> Points in Polygons.

That will create a new layer with an additional attribute column with the numer of points inside each polygon.

See this tutorial for assistance:

http://www.qgistutorials.com/en/docs/points_in_polygon.html

Related Solutions

QGIS – Counting Features Within Another Feature Using Aggregate Function

This expression will count the number of populated places for each country:

aggregate(layer:='ne_110m_populated_places',
          aggregate:='count',
          expression:=name,
          filter:=contains(geometry(@parent), $geometry)
          )

you can use the same expression to get the sum, just change the aggregate to 'sum' and the expression to the field name to summarize.

QGIS – Retrieving Attribute Field from Polygon Feature Containing Point Feature

Firstly make sure that your layers spatially overlap i.e. the same location and the same CRS.

Secondly as it says in the documentation for aggregate() function:

The source feature can be accessed with the variable @parent.

So, you can try either this:

aggregate(
    layer:='polygon layer's name',
    aggregate:='max',
    expression:="polygon layer's field name", 
    filter:=contains($geometry, geometry(@parent))
    )

or this:

aggregate(
    layer:='polygon layer's name',
    aggregate:='max',
    expression:="polygon layer's field name", 
    filter:=within(geometry(@parent), $geometry)
    )

Alternatively you can achieve the desired output with the overlay_within() and array_max()functions:

array_max(
    overlay_within(
        layer:='polygon layer's name',
        expression:="polygon layer's field name"
        )
    )

Best Answer

Related Solutions

QGIS – Counting Features Within Another Feature Using Aggregate Function

QGIS – Retrieving Attribute Field from Polygon Feature Containing Point Feature

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