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

Andreas Laggner andreas.laggner at fal.de
Thu Aug 16 02:04:17 PDT 2007

Moin Robert,

thanks for your reply. I use this statement now:

create table atkisvegddis(oba integer, land character varying(2)) with oids;
select addgeometrycolumn('','atkisvegddis','geom','31467','MULTIPOLYGON',2);
insert into atkisvegddis
    select oba, land, memgeomunion(geom) from atkisvegd
    group by oba, land;

it seems this function takes some time.....atkisvegd has 7,2 million 
polygons (postgis takes one hour so far and still working) or did i 
something wrong?

hasta luego         Andreas

Burgholzer,Robert schrieb:
> 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/ 

More information about the postgis-users mailing list