2

I have two layers which are perfectly nested. There are dozens of SA1 polygons (thin grey outline - see image attached) within the LGA polygon (thick green outline - see image attached). I would like to select all SA1s within the larger LGA polygon.

When I try and do this with the "select by location" tool by choosing "are within" it excludes SA1s where the centroid is clearly within the LGA boundary but may slightly intersect. When I choose "are within" and "intersect" it then spills over the LGA boundary.

The second image shows the ideal end state of what I am trying to achieve.

This is a very simple task in MapInfo using the boundary selection tool but is much harder in QGIS.

PROBLEM BEING DESCRIBED

IDEAL END STATE

3
  • 2
    if you want to use the centroids then you need to tell it to use them, it is currently using the whole polygons and the boundaries are not within the polygon.
    – Ian Turton
    Commented Aug 17, 2023 at 7:52
  • "are within" would work if your smaller polygons are perfectly snapped to the bigger ones, I checked on administrative polygons I have at different hierarchical levels and it works just right.
    – Kalak
    Commented Aug 17, 2023 at 8:18
  • 1
    related post
    – JGH
    Commented Aug 17, 2023 at 10:14

1 Answer 1

1

So if I understand correctly the way I'd get the selection you're after is by using "Intersection" on the two polygon layers while you have the one larger polygon area selected.

Starting Image Starting image

1 Step; Select the larger polygon and run Intersection, Larger polygon as input with selected futures only, overlay layer smaller polygons that overlap the larger polygon. Intersection

Final overview of the intersection; As you can see the polygons are not exceeding the outline of your larger polygons and will inherit the attributes of the previous polygons. Final overview

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.