[postgis-users] ST_Union results in empty polygon

Wed Apr 11 10:06:54 PDT 2018

Thanks for the response -- after I sent in my original question I was also able to pinpoint the issue to ST_Buffer.  I confirmed that the input polygon is valid, and from what I can tell, this polygon has a very thin portion running along the "edge" of an area by which it has been clipped.  This polygon when buffered by a very small amount, returns the empty polygon.

I'm assuming that something in the ST_Buffer process is hitting an error (Self-intersection? Some other type of topology exception?) which returns an empty polygon.  However, unlike with other operations (ST_Union in particular), PostGIS did not print a warning or error message, which made troubleshooting difficult.  Additionally, I'm still not sure why buffering by a tiny amount on a valid geometry would create an invalid geometry, but I recognize we're dealing with extreme precision for the nodes.

I have confirmed the same behavior when running an identical buffer operation on the same polygon using the Python "shapely" module, so this looks to be GEOS rather than PostGIS specific.  Should I reach out to that team to either track down the source of the issue or request that the API print a warning when this operation hits an error?

Thanks,
Jon

From: postgis-users <postgis-users-bounces at lists.osgeo.org> on behalf of Birgit Laggner <birgit.laggner at thuenen.de>
Reply-To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Date: Thursday, April 5, 2018 at 11:01 AM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: Re: [postgis-users] ST_Union results in empty polygon

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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