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I have 2 shapefiles that cover a similar region, a river catchment and a hex-grid of surface water bodies within the catchment. The catchment has subcatchments, and I'm trying to link the hex-grids that overlay each sub-catchment in a column in the attribute table.

Using the geometry expressions in the Field Calculator in QGIS 3.28.4 LTR, I have got an expression that covers hexes that lie within (overlay_within), but I also want to include the hexes that Overlap (Overlap) in the expression.

I've been using the QGIS Documentation to understand the expression I want to write (Select By Location and 'Overlay_' geometry expressions), but my results only show the ones within, the ones that cross, or the ones that intersect, not only the first two that I want.

I've illustrated what I mean; enter image description here

I want the expression to list in the hex-grid attribute table that Circle 1 AND Circle 2 are spatially connected to the Orange Box. Circles 3 and 4 would be spatially connected to Box 2.

At the moment I only have 2 separate expressions

array_to_string(overlay_within('Box',Box_ID)) which returns

Circle ID Connected to Box_ID
1 Box 1
2 NULL
3 NULL
4 Box 2

I can't have NULL values for shared-overlapping circles (hexes), because then they won't be factored in at all for later spatial operations.

array_to_string(overlay_intersects('Box',Box_ID)) which returns

Circle ID Connected to Box_ID
1 Box 1
2 [Box 1, Box 3]
3 [Box 1, Box 2]
4 Box 2

I can't have arrays or multiple spatial relations for one Circle (Hex) because that will cause issues later down the line.

overlap I found I couldn't use because I couldn't get it to include only the Box_ID attribute, just the entirre shapefile name (i.e. All the Circles are connected to 'Boxes.shp').

What I want is just overlay_within and overlap (or intersect minus touch) that produces this result below with

Circle ID Connected to Box_ID
1 Box 1
2 Box 1
3 Box 2
4 Box 2

How would I construct the expression for what I'm specifying, or is this a multiple and separate expression operation?

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    Does using the centroid will work ? In your exemple the centroid will be the center of the circle (so circle 2 centroid will fall in box 1) but with very irregular feature you may get case where the centroid fall in the wrong box
    – J.R
    Apr 27, 2023 at 13:31
  • @J.R I'm using identical sized hexes, so the centroids might work! I'll figure out if I can do that on a per-hex basis within the Field Calculator and then I can try that with overlay_within. Apr 27, 2023 at 14:59
  • So I've tried array_to_string(overlay_intersects('Box_Layer', Box_ID, centroid($geometry))) and while it does check out, it only returns a Null values for all features. Similar results with array_to_string(overlay_contains('Box_Layer', Box_ID, centroid($geometry))) Apr 27, 2023 at 17:11

1 Answer 1

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Try this:

overlay_intersects('Box_Layer', Box_ID, sort_by_intersection_size:='des')[0]

This code will get all intersecting boxes ID's and sort them in an array by size of intersection with descending order (biggest intersections first). Then [0] index takes first array's element - it's the box with biggest common area with the circle.

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  • Thanks, I tried this and it appears to have the same issue - The syntax checks out within the Field Calculator, but it only produces NULL results. Similar if I repeat it with array_to_string(overlay_intersects('Box_Layer', Box_ID, sort_by_intersection_size:='des')[0]) or if I put double quotes around the Box_ID ("Box_ID"). Does it matter very much if the Box ID of the Box and Circle's 'Box_ID' attribute is an integer? Apr 28, 2023 at 9:01
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    Update: I accidentally used the wrong layer without the Box_ID set, hence the Null Value. This actually worked perfectly! Thank you very much. Apr 28, 2023 at 9:50

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