[GIS] Determining which polygon(s) each point lies in using QGIS

overlaypointpoint-in-polygonpolygonqgis

I am using QGIS software. I have shapefile layer of points and a shapefile layer of polygons(many of them overlapping). I need to determine which polygon(s) each point lies in.

I tried "Join attributes by location" with the solution posted here:(How to find the polygon inside which a point lies?). My question is different because many of my points will be inside multiple polygons. This only gives one polygon per point. My solution will have many polygons for each point.

For example, I would like to know the identity of every blue circle that lies underneath each black dot.

How can I complete this analysis?

Best Answer

I solved this problem. Simply use the "Intersect" feature under Geoprocessing tools.

Explanation :

Target : points layer, Join layer: polygons layer. That way info from your districts goes into your points
Geometric predicate : Check ON all the options: intersects, touches etc. I haven't looked further into this yet but just one option had not worked out earlier.
Attributes summary : Take attributes of first located feature
Joined table : Keep all records ... that will preserve the points that were outside of all the wards too
Get rid of everything in the "Statistics" option

This will create and add another layer which has all your points as they were, but now the attributes have extra columns from the polygon layer. So this is kind of like the "left join" in python-pandas. The "keep all records" option ensures that points falling outside of all polygons are not lost.

To export this data back into CSV format, I prefer using the MMQGIS plugin and Export-Geometry-to-CSV option in that.

QGIS – Selecting Only One Point within a Polygon

If you don't care which point is selected, you can use this small PyQGIS script. Paste into a blank editor in the Python Console, change the layer names in the first two lines to match your layer names and click run.

# Change layer names in the first two lines to match your layers...
point_lyr = QgsProject.instance().mapLayersByName('New scratch layer')[0]
poly_lyr = QgsProject.instance().mapLayersByName('Buffered')[0]

ids = []

for feat in poly_lyr.getFeatures():
    contained_point = [f for f in point_lyr.getFeatures() if f.geometry().within(feat.geometry())][0]
    ids.append(contained_point.id())

point_lyr.selectByIds(ids)

Result:

Best Answer

Related Solutions

QGIS – Assigning Polygon Names to Contained Points Using Shapefile

Explanation :

QGIS – Selecting Only One Point within a Polygon

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