[postgis-users] GEOS Problems

[Rene Neidt]

Hello Postgis Users,
 
we are building a complex application with many gis operations.
The most problems occured within the geomUnion, intersection and difference functions.
 
We have some different problems using GEOS operations. We think that is a 
problem with tolerances while processing the geometries.
 
We have tried "SnapToGrid","Buffer" with zero distance and so on ... , but we can't find a
solution that will work in all cases.
 
Either we get an Error from Geos (eg. "No outgoing edge found" and many more), or we dont get the 
expected result.
 
The following example creates two different results for the same operation with the
"same" geometries, but different geometrie casts.
 
----- import with psql -----------------------------------------------
 

CREATE TABLE t1 (
    geom public.geometry
);
COPY t1 (geom) FROM stdin;
01030000000100000008000000F326BCE75EA9544154B30FE548955541D31C05585EA9544149A605194A9555413D0AD7B35CA9544185EB5118489555415C8FC2655CA95441333333C347955541295C8FD25CA9544185EB516847955541EC51B81E5DA954410AD7A3C0479555410AD7A3A05DA954416666665647955541F326BCE75EA9544154B30FE548955541
01030000000100000005000000D41C05585EA9544148A605194A955541F326BCE75EA9544154B30FE5489555418FC2F5685FA95441295C8F8249955541C3F5287C5EA9544152B81E454A955541D41C05585EA9544148A605194A955541
\.
 
-- the statement below will produce a single-polygon with 10 Points, 
-- this is the expected result ...
select asText(geomUnion(asText(geom))) from t1
 

-- but this statement produces a multipolygon with 13 points 
select asText(geomUnion(geom)) from t1 
 
 
-- Postgres Version : "PostgreSQL 8.1.2 on i686-pc-mingw32, compiled by GCC gcc.exe (GCC) 3.4.2 (mingw-special)"
-- Postgis Version  : "POSTGIS="1.1.1" GEOS="2.2.1-CAPI-1.0.1" PROJ="Rel. 4.4.9, 29 Oct 2004" USE_STATS (procs 1.1.0 upgrade)" 
-- OS: Windows Server 2003
 
-- Best regards 

 __________________________________________________________________

Rene Neidt
Projektleiter Software
IDU Ingenieurgesellschaft für Datenverarbeitung und Umweltschutz mbH
Theodor-Körner-Allee 16
02763 Zittau
Germany

Tel ++49 3583 61-15-37
Fax ++49 3583 61-12-10

Internet http://www.idu.de/, http://www.webmapserver.de/
Anfahrt: http://www.stadtplan-zittau.de/?alias=idu

 
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