QGIS – Selecting Polygons Completely Inside Another Layer Without Common Vertices or Edges

qgisselect-by-location

Backgound:

At 3.34.2, consider a scenario involving two polygon layers, as shown in the diagram below. The task is to select by location polygons from Layer A, compared to Layer B, that return only polygon #3.

Polygon #3 is unique because it is completely within a Layer B polygon, yet it shares no vertices or edges with that polygon.

I've tested every checkbox option in the Select by Location tool. None of them will return just polygon #3. For example, the "are within" option selects all three polygons.

I've also experimented with the "overlay" expressions, such as overlay_within('Layer_B'), but without success.

I also tried the "Select Within" plugin, but it was not able to select just Polygon #3.

Question:

How to select polygons from one layer that are completely inside polygons from another layer, yet have no vertices or edges in common.

Bonus points for techniques that would select Polygon #1, as well as Polygon #2!

Best Answer

You could add an extra clause to check if the polygon from Layer A intersects the boundary (a line) of Layer B.

overlay_within('Layer B') 
and not 
intersects(@geometry, boundary(aggregate('Layer B', 'collect', @geometry)))

Related Solutions

QGIS – How to Select Features Intersecting Another Layer’s Selection

VirtualLayers possibly to the first world problem rescue; this is a nice solution if you can express your polygon selection with some condition (I use an id <some_id> as an example here):

Layer -> Add Layer -> Add/Edit Virtual Layer...
Import -> both your WFS layers when loaded
(you can try adding the service directly via Add -> select WFS as provider and paste in the URL, but it keeps on throwing errors for me on QGIS 2.18)

and then type

SELECT pts.*
FROM <point_layer> AS pts,
     <polygon_layer> AS plg
WHERE plg.<id_column> = <some_id>
  AND ST_Intersects(pts.geometry, plg.geometry);

into the Query field.

If you want to manually select features:

Edit -> Copy Features
Edit -> Past Features As -> Temporary Scratch Layer...

and continue with your usual workflow from there
(you could of course use that layer in the above query to get the intersecting points into a virtual layer; just omit plg.<id_column> = <some_id> AND in above query and replace <polygon_layer> with the name of the scratch layer).

QGIS – How to Subset Shapefile Features Based on Another Shapefile’s Position

In case the two layer do not exactly match, so that the polygons of the layer with the smaller polygons (lets call it p2) at the borders overlap with more than one of the larger polygons of the other layer (p1), you can use the centroid of the smaller polygons and get the $id of the larger polygon that it is within. Use field calculator to create an attribute field called within with this expression. Then proceed as in step 2 of the other solution:

array_get (
    overlay_nearest('p1', $id, limit:=-1),
    array_find( 
    array_foreach (
        overlay_nearest('p1', $id, limit:=-1),
        within (
            centroid ($geometry),
            geometry (
                get_feature_by_id (
                    'p1',
                    @element))))
        ,
        1))

As you can see (e.g. at the highlighted spot), small poygons that overlap with two of the larger polygons are assigned to the side where the centroid is within:

Edit

Tested with the dataset you provided in the comment. To get proper results with this data, use this expression (apply it on the layer with the small polygons; the layer with the larger polygons should be named p1):

array_max(
    array_foreach (
        overlay_intersects('p1',$id),
        if (
            within (
                centroid ($geometry), 
                geometry (get_feature_by_id ('p1', @element))
            ),
            @element,
            ''
        )
    )
)

Then use Split vectort layer based on the field created in this way as explained in my other solution.

Best Answer

Related Solutions

QGIS – How to Select Features Intersecting Another Layer’s Selection

QGIS – How to Subset Shapefile Features Based on Another Shapefile’s Position

Related Question