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Background: Sometimes in modelling hydrology (e.g. in SWMM) we have runon from one subcatchment area to another. These subcatchments are records in a shapefile. I want to show this situation as a line from the centroid of one polygon to another (the "outlet"). This is what I tried (which obviously doesn't work):

select make_line(st_centroid(Subcatchments.geometry),st_centroid(Subcatchments.geometry))
from Subcatchments
where Subcatchments.OUTLET = Subcatchments.Name

Can anybody assist me in how to do this correctly?

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1 Answer 1

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The trick is to join the layer to itself:

SELECT make_line(st_centroid(s1.geometry),st_centroid(s2.geometry))
FROM Subcatchments s1
JOIN Subcatchments s2 
 ON s1.OUTLET = s2.Name
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  • Thanks! This was exactly what I was trying to do–I just couldn't quite figure out how. Commented Feb 2, 2024 at 23:00

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