QGIS Selecting Electoral Division – Selecting All Electoral Divisions Within Local Electoral Area in QGIS

overlapping-featurespolygonqgisselect-by-locationwithin

I have two vector layers:

electoral divisions (blue)(similar to counties)
local electoral areas (green outline) (similar to states).

I'd like to select all EDs 'within' a state using 'Select by Location' however 'within' doesn't select the EDs which touch the borders of the LEA (see selected EDs in yellow).

I'd eventually like to export the attribute table as a lookup detailing the EDs that sit within each LEA.

Best Answer

Use select by expression with this expression. It creates a small buffer around the electoral areas to be sure that all electoral divisions are completely within:

within (
    $geometry,
    overlay_nearest (
        'polygon',
        buffer ($geometry, 1)
    )[0]
)

polygon on line 4 is the name of the electoral areas
1 on line 5 is the buffer's distance

Related Solutions

[GIS] Selecting Counties within Region via Spatial Query using QGIS/PostGIS

PostGIS has a function that addresses just this centroid problem: use ST_PointOnSurface() to return a point guaranteed to be inside your county, then just find the ST_Intersection() of the county-points and region. Like this:

SELECT * FROM counties c, regions r WHERE
ST_Intersects(ST_PointOnSurface(c.the_geom), r.the_geom);

[GIS] Constrained Voronoi polygons QGIS

It's a multi-step process, so you should create a custom model or script. In order to avoid slivers of polygons that belong to neighboring polygon points, you'll need to iterate through your states layer. Working with state polygons and a set of random points, I'll go through the process so that you can understand what's going on inside our model.

Inside the Model

Extract by location
- Extract features from: points_layer
- Where the features: intersect
- By comparing to the features from: polygons_layer

Voronoi polygons
- Input layer: 'Extracted (location)' from algorithm 'Extract by location'
- Buffer region: 500 (It seems excessive, but we want to ensure that your Voronoi polygons completely cover the state they're in. Since we'll clip this layer in the next step, it's irrelevant how big we make this.)
- Parent algorithms: Extract by location

Clip
- Input layer: 'Voronoi polygons' from algorithm 'Voronoi polygons'
- Overlay layer: polygons_layer
- Clipped: output
- Parent algorithms: Extract by location and Voronoi polygons

As you can see, we have our state-constrained Voronoi polygons in which points from neighboring states do not affect their arrangement.

Here's an image of the model:

The image doesn't convey this, but note that the polygon layer input of the model is a Vector Features object, not a Vector Layer.

Running the Model

When you go to run your model it is vital that you click on the icon to iterate through the features of your States layer.

Once you've run the model, you can simply run Merge vector layers on the outputs to get a single-layer output, if that is important to you. Unfortunately, I'm not aware of an easy way to merge multiple model outputs within the model itself (please, someone correct me if there is something).

If you're comfortable with Python, you can easily modify the exported script of this model to handle that additional step. As you did not mention code-based solutions in your question, however, I will leave that out.