Hi, I thought I had everything figured out with respect to my problems unioning and differencing polygons, but I am running into more TopologyExceptions that have me confused.
After looking at Paul Ramsey's FOSS4G presentation, I realized that what I really wanted to do was an "ArcGIS Union Operation" - i.e. reducing a set of overlapping polygons into all possible combinations of overlaps. My table consists of a set of MULTIPOLYGONs that sometimes overlap. I used ST_Buffer(0) to force them to be valid polygons (and checked this worked correctly). Then I did the following to reduce every polygon to just the LINESTRINGs that define them: SELECT ST_ExteriorRing((ST_DumpRings((ST_Dump(wkb_geometry)).geom)).geom) AS geom INTO TEMP TABLE ht1 FROM my_geo_table; Note that I added ST_ExteriorRing to ST_DumpRings to reduce polygons to linestrings. Is this correct? Then I wanted to union these linestrings to assure that they have all the nodes at overlaps, but I keep getting a TopologyException: SELECT ST_Union(geom) AS geom INTO TEMP TABLE ht2 FROM ht1; NOTICE: TopologyException: found non-noded intersection between LINESTRING (-165.443 23.1836, -165.443 23.1836) and LINESTRING (-165.443 23.1836, -165.464 23.2079) at -165.443 23.1836 ERROR: GEOS union() threw an error! Presuming the coordinates given are actual geographic coordinates, it appears that this problem occurs at a location that two different linestrings just barely overlap (see attached image). Is this a postgis / GEOS problem or is there something that I am doing that is conceptually wrong? In either case, is there a way to move forward? The next steps in the process would be to use ST_Polygonize to get back a bunch of polygons, ST_PointOnSurface to find interior points and ST_Intersects to find the original polygons these points belonged to. Thanks again for the help. Cheers, David On Mon, 2010-12-20 at 17:49 +0100, Peter Hopfgartner wrote: > AFAIK there are slightly different definitions of valid polygon between shape > files and PostGIS/Simple Features. > Paul Ramsey had a talk on a number of tricks to correct those problems (and > some more): > http://2010.foss4g.org/presentations_show.php?id=3369. > > Peter Hopfgartner > > R3 GIS Srl - GmbH > http://www.r3-gis.com > > > --------David Kaplan <david.kap...@ird.fr> wrote-------- > Subject: [postgis-users] what to do about TopologyExceptions > Date: 20.12.2010 09:54 > > >Hi, > > > >I am working with a GIS table that is derived from importing a fairly > >complex shapefile. I want to do a number of union and difference > >operations, but I am encountering TopologyException problems and am > >wondering how much attention I need to pay to these exceptions and what > >is the best way to deal with these problems. > > > >For example, upon doing a simple union operation, I have: > > > >CREATE TABLE by_country_iucn AS > >SELECT min(ogc_fid) AS ogc_fid, country, iucn_cat, > > ST_Multi( ST_Union( wkb_geometry ) ) AS wkb_geometry > >FROM pol_2010 > >GROUP BY country, iucn_cat > >ORDER BY country, iucn_cat; > > > >I get the following exception at the end: > > > >NOTICE: TopologyException: side location conflict at 29.1972 0.251105 > > > >However, I am not sure what to do with this for two reasons: > > > >1) The point indicated in the exception (presuming this is a 2D point > >location) is located in the middle of nowhere and none of my polygons > >come anywhere near it (the nearest is >1000 km away). > > > >2) Looking at the result of the union, I don't see any obvious problems. > >Do these exceptions mean I am missing some part of the desired union? > > > >For the union, this returns a result no problem, but for the difference > >operations that follow it, the query fails with no result after a > >similar exception. > > > >I saw in a previous thread that I should use ST_IsValid to test for > >problems. I applied this to my shapefile and it found lots of results. > >Looking at the "bad" polygons, many look fine to the naked eye, but some > >do have strange things like internal lines. Is there a good way to fix > >these? ST_SimplifyPreserveTopology? Will doing this help the other > >union and difference manipulations? > > > >Thanks for the help. > > > >Cheers, > >David > > > >-- > >********************************** > >David M. Kaplan > >Charge de Recherche 1 > > > >Institut de Recherche pour le Developpement > >Centre de Recherche Halieutique Mediterraneenne et Tropicale > >av. Jean Monnet > >B.P. 171 > >34203 Sete cedex > >France > > > >Phone: +33 (0)4 99 57 32 27 > >Fax: +33 (0)4 99 57 32 95 > > > >http://www.ur097.ird.fr/team/dkaplan/index.html > >http://www.amped.ird.fr/ > >********************************** > > > > > >_______________________________________________ > >postgis-users mailing list > >postgis-users@postgis.refractions.net > >http://postgis.refractions.net/mailman/listinfo/postgis-users > > > -- ********************************** David M. Kaplan Charge de Recherche 1 Institut de Recherche pour le Developpement Centre de Recherche Halieutique Mediterraneenne et Tropicale av. Jean Monnet B.P. 171 34203 Sete cedex France Phone: +33 (0)4 99 57 32 27 Fax: +33 (0)4 99 57 32 95 http://www.ur097.ird.fr/team/dkaplan/index.html http://www.amped.ird.fr/ **********************************
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