Counting points in polygon using PyQGIS

Question

Is there any simple integration for calculating count of points in polygon using QGIS-Python integration?

In other words, are there any pre-defined functions for points in polygon of QGIS using python, so that we can use them in our python script?

Answer 1

Great methods already posted! There are also tools from the Processing Toolbox which you could call from your script (this is assuming you are running scripts inside QGIS):

• Count points in polygon
• Counts points in polygon (weighted)
• Count unique points in polygon

Taking the first tool, you could call it using:

import processing

Result = "path\to\saved_output.shp"
processing.runalg("qgis:countpointsinpolygon", "path\to\polygon_layer.shp", "path\to\point_layer.shp", 'NUMPOINTS', Result)

where NUMPOINTS is the name of the field which stores the number of points.

I'm using QGIS 2.8.2-Wien, Processing plugin v2.9.3. Hope this helps!

Answer 2

I rewrote my answer from a parallel question counting points in current atlas feature: How to count points within the current Print composer atlas feature in QGIS 2.8?

Build a new function in the expression editor:

from qgis.core import *
from qgis.gui import *
from qgis.utils import iface


@qgsfunction(args="auto", group='Custom')
    def countPointsInPolygon(pointLayerName, geom, feature, parent):
        # If point geom is empty, return 0
        if (geom is None):
            return 0

        # Get point layer reference from layername
        pointLayer = QgsMapLayerRegistry.instance().mapLayersByName(pointLayerName)[0]

        # Raise if layer not found
        if pointLayer is None:
            raise Exception("Layer not found: " + pointLayerName)

        # Count point within current polygon feature
        countPoint = 0
        for pointFeature in pointLayer.getFeatures():
            pointGeom = pointFeature.geometry()
            if (pointGeom is None):
                continue
            if pointGeom.within(geom):
                countPoint += 1

        return countPoint 

Use it as a expression on the polygon layer where 'mypoints' is the name of the point layer: countPointsInPolygon( 'mypoints', $geometry )

If you don't want it implemented as an expression, you can call the function from your own script looping the features of the polygon layer calling countPointsInPolygon for each feature.

Answer 3

the most simple way is to use "Join attribute by location". You can do it from the menu (data managment > Join by location), or with Python ("qgis:joinbylocation") with the option "take summary of intersecting features", computing the sum of a field with "1" at each point.

Answer 4

I have never worked with QGIS's python API, so my answer is a wild guess. But it looks like QGIS is using GEOS geometry objects similar to django. So this could work in QGIS too:

len(geom.coords)

Answer 5

I assume your polygon is a qgsfeature :

http://qgis.org/api/classQgsFeature.html

I didn't test it but I guess it would work :

len(polygon.geometry().asPolygon())

polygon.geometry() gives a qgsgeometry class.

qgsgeometry.asPolygon() gives a list of the point of your polygon.

Edit : You want the points inside the polygon? You have points in an other layer and wonder which ones intersect your polygon?