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I'm making the transition from GIS to PostGIS and am stuck on this spatial query.

I have background geometry (purple, outlined red) which has an overlay of islands (blue, below); I need the area of only the islands within the red line. In GIS this would be a clip, using the purple shape as my mask.

background purple shape

blue islands

So I query using ST_Within() and obtain the green islands, which is great, but - logically - I'm missing those which are not fully within the mask: st_within result

And I can still find those islands overlapping the boundary of my polygon using ST_Overlap() to find these pink polygons:

st_overlap results in pink overlap polygons

But is there an efficient way to get the area of the islands only within the red outline? This will cut a percentage of the island polygons, I understand, those will be associated with the adjacent background polygon.

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  • From which GIS is it that you are transitioning?
    – Vince
    May 2, 2019 at 15:10
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    You are looking for the ST_Intersection function postgis.net/docs/ST_Intersection.html
    – ziggy
    May 2, 2019 at 15:13
  • Thanks @ziggy! I'd looked at that part of the docs previously, but didn't understand the st_intersection(), st_intersects() relationship as @pdavis clarified below. While it lasts, this link may help others like me who failed at RingTFM: bostongis.com/postgis_intersection_intersects.snippet
    – Jeff
    May 3, 2019 at 8:17

1 Answer 1

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This is what you're looking for:

SELECT ST_Intersection(t1.geom, t2.geom)
FROM t1 JOIN t2 ON ST_Intersects(t1.geom, t2.geom)

For more info: https://postgis.net/docs/ST_Intersection.html

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