QGIS Polygon Selection – Selecting Only Polygons Containing Another Polygon with Select by Location

overlapping-featurespolygonqgisselect-by-expressionselect-by-location

In QGIS 3.16.11 on Windows 10 I have a layer with single polygons.

Given this polygon layer with, I'm trying to select only the polygons that that contain at least one other polygon (A & B)

My first thought was, gonna be easy just do:

Research tools -> Select by Location
```
INPUT: original_layer  
INTERSECT: original_layer  
PREDICATE: contain   
```
But everything got selected. It's acting like contain select polygon that contain but also that are equal. same thing happens with are within.
Then I tried with intermediary layer of slightly inflated polygons and to do the contain on that layer

Geoprocessing Tools -> Buffer
```
INPUT: original_layer  
DISTANCE: 0.00001
OUPUT: inner_polygon_slighly_inflated  
```
Research tools -> Select by Location
```
INPUT: original_layer  
INTERSECT: inner_polygon_slighly_inflated  
PREDICATE: contain
```
Then I got selected B, but not A.

Can't find a way to this and which seems quite simple. Is there a way to make a PREDICATE: contain but not equal?

Best Answer

Use "Select by expression" with this expression: overlay_contains(@layer).

Related Solutions

[GIS] Select polygons by attribute belonging to another layer

Depending on your desired output you can utilize 2 features of the Select by location tool in QGIS (Vector > Research Tools > Select by Location). In the image below you will see the options for 'use selected features only' and 'modify current select by...'.

In your example, first select your desired municipality polygon (from Shapefile B - you can do this manually or through a query) and then select 'use selected features only' when selecting your land cover polygons (Shapefile A) using the Select by location tool. This will result in only those land cover polygons being selected for a single municipality.

enter image description here

If desired you can then append additional selections using the 'add to current selection' option.

[GIS] QGIS select by location doesn’t use selected features only

Not sure if still actual but recently I have found a solution with no settings correction. I needed to iterate some selected features and get all intersections from other layers. Then make a pack of folders with same layers containing different features.

When you want to specify only selected option the processing script is written in a bit another way. Firstly you have to get an id string of your layer. Then put that string into a processing script but wrapped in QgsProcessingFeatureSourceDefinition with True parameter and then you are ready to perform a selection using only selected input features.

import processing 

# getting a layer with selected features with 'FrameLayer' project name 
lName = "FrameLayer"
lNameVL = list(filter(lambda x: x.name() == lName , list(QgsProject.instance().mapLayers().values())))[0]   

# getting its id string
lNameId = lNameVL.id()  

# selecting features from active layer intersecting lNameVL layer using 'only selected' option 
processing.run("qgis:selectbylocation",  {'INPUT': iface.activeLayer(), 'PREDICATE': 0, 'INTERSECT':QgsProcessingFeatureSourceDefinition(lNameId, True),  'METHOD':0})

Best Answer

Related Solutions

[GIS] Select polygons by attribute belonging to another layer

[GIS] QGIS select by location doesn’t use selected features only

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