[postgis-tickets] [PostGIS] #4182: Unexpected TopologyException during ST_Union aggregate function with valid polygons

[PostGIS]

#4182: Unexpected TopologyException during ST_Union aggregate function with valid
polygons
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  Reporter:  blaggner  |      Owner:  pramsey
      Type:  defect    |     Status:  new
  Priority:  medium    |  Milestone:  PostGIS GEOS
 Component:  postgis   |    Version:  2.3.x
Resolution:            |   Keywords:
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Comment (by fengel):

 I attached my safe_union. It's probably pretty far from optimal, because I
 am more a user than a developer. I was glad that my first custom aggregate
 function worked at all. Maybe you can give me some hints to make it
 better.
 I am not sure about the best methods to attemt to fix exceptions. In
 LostGIS the main approaches seem to be translating, buffering by a small
 value (but not buffering back with a negative value?) and force_rhr. In my
 function I tried snapping to different precisions. I have the impression
 that at least the snapping doesn't succeed to fix things very often.
 The main benefit for me is that exceptions are skipped. I plan to write
 the point coordinates and geometries of the unfixed exceptions to a
 (temporary) table so that I can check visually in my GIS-software where
 exeptions occured and if the output has significant gaps compared to the
 original polygons. A result with minor gaps would be a "reasonable
 result". But of course it is a subjective decision whether a result is
 still acceptable or not. As you said, it also depends on the intended use.
 So far using LostGIS and my functions the results are ok on that account.
 I often check total area before and after and decide if I can live with
 the result. I usually use the result for aggregating area sums by factors.
 So if the result is slighly wrong because there is 1% of the original area
 missing I can usually live with that.\\

 Also I want to point out that I am probably not using the latest versions
 of PostGIS and GEOS. I should probably upgrade but my operating sytem and
 Postgres needs upgrade, too, and my time is limited at the moment.\\
 {{{
 #!sql
 select postgis_full_version()
 }}}
 POSTGIS="2.4.2 !r16113" PGSQL="95" GEOS="3.5.1-CAPI-1.9.1 !r4246"
 PROJ="Rel. 4.9.2, 08 September 2015" GDAL="GDAL 2.2.2, released
 2017/09/15" LIBXML="2.9.3" LIBJSON="0.11.99" LIBPROTOBUF="1.2.1" RASTER
 {{{
 #!sql
 select version()
 }}}
 PostgreSQL 9.5.19 on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu
 5.4.0-6ubuntu1~16.04.10) 5.4.0 !20160609, 64-bit

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/4182#comment:13>
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