Select by location gives no result in QGIS?

Question

I am using QGIS 2.18 and I have two shapefiles:

1. "circle" layer; containing lots of circular polygons, many of them overlapping.
2. "mask" layer; containing few much larger polygons (not overlapping).

The mask layer covers many, but not all of the circles. I tried to select all polygons of the circle layer which lie within the boundaries of the mask layer-polygons. Both layers share the same CRS. However, both the standard QGIS-function select by location and the GRASS-function v.select produce no result. The "intersect" tool does not help here, as circles at the edge of the mask layer get cut in pieces. Why is this? Any solutions to this issue?

Answer 1

I was able to get this to work (in 2.18.3), although I confess it took me a couple of tries ;-)

You've checked the CRSes so it's not that - if the CRSes didn't match you'd usually get a warning anyway.

You need to make sure you put the layers the right way round. This first is the layer you want the selection to come from (buffered circles layer) and the second is the layer you want to use as the mask.

Second, you need to use within, not contains, like so

this makes sense, you want the buffered circles within your polygons, not the buffered circles containing your polygons (which won't happen in your case, as the polygons are much bigger than the circles). My first attempt was to use 'contains', which gave me the empty selection you saw.