[GIS] PostGIS equivalent to ArcGIS Select by Location “Are within”

postgisselect-by-location

I am trying to select polygons that are within a larger polygon in PostGIS but those who share boundaries with the larger polygon are not being returned.

If I use ArcGIS Select By Location I get the result I need, 17 features. But if I use PostGIS I get only 4 features, the same as if I use ArcGIS "are completely within".

Correct result in ArcGIS

Queries that I have tried, but without success:

select count(a.cd_geocodi)
from t_ibge_setor_censitario a, t_ibge_municipio_2015 b
where b.nm_municip = 'PARAÍSO DAS ÁGUAS'

cand ST_Intersects(a.geom,b.geom);
Result: 34

select count(a.cd_geocodi)
from t_ibge_setor_censitario a, t_ibge_municipio_2015 b 
where b.nm_municip = 'PARAÍSO DAS ÁGUAS'
and ST_Within(a.geom,b.geom);

Result: 4

select count(a.cd_geocodi)
from t_ibge_setor_censitario a, t_ibge_municipio_2015 b
where b.nm_municip = 'PARAÍSO DAS ÁGUAS'
and ST_Contains(b.geom,a.geom);

Result: 4

select count(a.cd_geocodi)
from t_ibge_setor_censitario a, t_ibge_municipio_2015 b
where b.nm_municip = 'PARAÍSO DAS ÁGUAS'
and ST_Coveredby(a.geom,b.geom);

Result: 4

select count(a.cd_geocodi)
from t_ibge_setor_censitario a, t_ibge_municipio_2015 b
where b.nm_municip = 'PARAÍSO DAS ÁGUAS'
and ST_Covers(b.geom,a.geom);

Result: 4

Is there a way to get all those 17 features in PostGIS?

Best Answer

This depends a little on whether the small features are dependent on the larger features: do they actually share a common boundary, or are there small discrepancies along their apparently common edges?

ST_Within(a.geom, b.geom): gives you the geometries in b that are fully within a.geom. And it means fully within.

ST_Intersects(a.geom, b.geom): this is a little more tolerant, returning anything in b that merely touches a at all, even at a single point on the boundary. But if you combine this with NOT ST_Touches(a.geom, b.geom) as mentioned in a comment by John Barça (ST_Intersects(a.geom, b.geom) AND NOT ST_Touches(a.geom, b.geom)), then you will be left with (hopefully) the result you want. Note that small imperfections in the boundaries might still mean that the result isn't adequate, and you might need some kind of tolerance to get the correct result. If you still have issues, you could apply a small negative buffer to the geometries in b to make them smaller (this is pretty dirty though), or use ST_DWithin(a.geom, b.geom, 10), or you could perhaps ensure that a.geom and b.geom have a verified topological relationship and don't differ slightly at the edges.

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