Nearest line to polygon based on equal attribute

Question

I have a polygon shapefile and a line shapefile with several thousand features each. I want to get:

The nearest distance between a polygon and a line that have the same attribute (e.g. ID). Example here (I want to get the green line as an output).

I have played around with various combinations of all distance tools in QGIS, like Distance to nearest hub, Join attributes by nearest, v.distance.

v.distance SHOULD be able to do what I want with 'upload' = 'to_attr', but I am getting no output.

Do you want an actual line geometry, or do you only need the distance value? — csk, Commented Aug 19, 2019 at 16:23
Just the distance value would be enough, though the actual line geometry would have the bonus of making it easier to check visually whether the results are correct. The answer from @snaileater generates the distance, which is sufficient for me. — wowbag, Commented Aug 20, 2019 at 8:46

Snaileater · Accepted Answer · 2019-08-19 17:52:44Z

1

I don't know about the tools you mention but you can try a query like this one :

select id, st_distance(poly.geometry, line.geometry) from poly, line
where poly.id=line.id

That should do the job ...

You input the query in QGIS DB Manager : You choose Database / Database Manager / Database Manager then Virtual Layers / Qgis Layers.

Adapt the query to your data sructure if needed ...

answered Aug 19, 2019 at 17:36

Snaileater

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Beautiful, it works! I had to remove 'id' from the query though, it gave me an error message about being ambiguous. So: select st_distance(poly.geometry, line.geometry) from poly, line where poly.id=line.id
– wowbag
Commented Aug 20, 2019 at 8:37
yep you have to mention either table ... so poly.id or line.id ... in that case they will be equal anyway ...
– Snaileater
Commented Aug 20, 2019 at 8:42

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