[postgis-users] what function to "dissolve" data?

[Burgholzer,Robert]

Andreas,
I have used the following query with varying degrees of success (that is, sometimes the memory requirements seem to be too much, and it takes forever to complete).  The resulting shape occasionally renders strangely in Qgis.

SELECT landuse_name, memgeomunion(the_geom)
GROUP BY landuse_name

Regular geomunion (instead of memGeomUnion) can also work, but seems to choke on larger collections.

r.b.

Robert W. Burgholzer
Surface Water Modeler
Office of Water Supply and Planning
Virginia Department of Environmental Quality
rwburgholzer at deq.virginia.gov
804-698-4405
Open Source Modeling Tools:
http://sourceforge.net/projects/npsource/
Web-Based Water Supply Planning Demo:
http://soulswimmer.dynalias.net/models/wsdemo/demo_hsi.php

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Andreas Laggner
Sent: Wednesday, August 15, 2007 12:30 PM
To: PostGis_Mailinglist
Subject: [postgis-users] what function to "dissolve" data?

Hi,

i want to simplify my data by dissolving lines (borders) between 
Polygons (the polygons share one line or at least one point) with the 
same attributes to get a smaller dataset and i did not find a function 
for that operation (so far). For example i have one million polygons 
with the same landuse but a different polygon_number or what else and i 
only want to distinguish between landuse in my data. Thus i can reduce 
my dataset to same hundred polygons (or perhaps even one big polygon if 
all polygons touches).

thanks for help                  Andreas



-- 
Dipl. Geoökologe Andreas Laggner
Institut für Ländliche Räume (LR)
Bundesforschungsanstalt für Landwirtschaft (FAL)

Institute of Rural Studies
Federal Agricultural Research Centre (FAL)

Bundesallee 50
D-38116 Braunschweig

Tel.: (+49) (0)531 596 5515
Fax: (+49) (0)531 596 5599
E-mail: andreas.laggner at fal.de
Homepage: http://www.lr.fal.de/ 

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