Selecting all electoral division within local electoral area in QGIS

Question

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.

Answer 1

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

Answer 2

I recommend installing the Select Within Plugin. It has the option to select the polygons where their centroids are located inside the polygons of the other layer.

Using this option, you can avoid any duplicates that occur when using the intersect predicate inside the Select by location tool.

Answer 3

One possible path is to create a new attribute with the centroids of the smaller areas, and then select all the points that are inside the larger polygon.