[postgis-users] ST_Union results in empty polygon

[Birgit Laggner]

Hi Jonathan, 

I tested your example and it seems the reason why the ST_Union results in an empty polygon is because the resulting polygons of the ST_Buffer are already empty. I could imagine complex polygons don't go well with a very small buffer. 

In my experience, ST_Union only results in empty polygons when the input polygons are not valid. Did you test if your polygons are valid? If validity is indeed the problem, you can solve the issue with ST_MakeValid rather than with ST_Buffer. 

Regards, 

Birgit 


Von: "Jonathan McCormack" <Jonathan.McCormack at fcc.gov> 
An: postgis-users at lists.osgeo.org 
Gesendet: Montag, 26. März 2018 20:05:42 
Betreff: [postgis-users] ST_Union results in empty polygon 



We've seen occasional inconsistencies from ST_Union where the resulting polygon is empty or clearly cannot be the result of the constituent geometries. These issues can often be resolved by applying a very slight buffer, though sometimes the reverse is true, where the ST_Union of the unbuffered geometries is fine, but applying the buffer causes the incorrect geometry. The following query shows this latter case (pardon the lengthy geometry strings): 



WITH poly AS ( 

SELECT 

ST_SetSRID( 

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geometry 

,4326) geom1, 

ST_SetSRID( 

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geometry 

,4326) geom2 

) 

SELECT ST_Area(geom1::geography) AS geom1_area, 

ST_Area(geom2::geography) AS geom2_area, 

ST_Area(ST_Union(geom1, geom2)::geography) AS g1g2_area_nobuffer, 

ST_Area(ST_Union(ST_Buffer(geom1, 0.0000000001), ST_Buffer(geom2, 0.0000000001))::geography) AS g1g2_area_buffer 

FROM poly; 



This query returns expected values for geom1_area, geom2_area, and g1g2_area_nobuffer, but returns 0 area due to an empty polygon for g1g2_area_buffer in two different PostGIS environments: 



POSTGIS="2.4.1 r16012" PGSQL="95" GEOS="3.6.2-CAPI-1.10.2 4d2925d6" PROJ="Rel. 4.9.1, 04 March 2015" GDAL="GDAL 2.2.0, released 2017/04/28" LIBXML="2.9.1" TOPOLOGY RASTER 

POSTGIS="2.4.3 r16312" PGSQL="100" GEOS="3.6.2-CAPI-1.10.2 4d2925d6" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 1.11.5, released 2016/07/01" LIBXML="2.9.7" LIBJSON="0.12.1" RASTER 



I'm aware that a buffer of 1e-10 decimal degrees is operating at a level of precision better suited for biology, but I still don't understand why PostGIS (or GEOS or wherever the issue lies) would return an empty polygon. 



Any ideas? Or are there better ways to work around these issues? 



Thanks, 

Jon 



Jonathan McCormack 


Attorney Advisor & Information Systems Specialist 


Auctions and Spectrum Access Division, WTB 

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