Moin Apostolis,
the union like ArcMap does it is much more complicated as it seems -
just one button in ArcMap :-) - and there are no good solutions in
PostGIS. We are working on that problem for 2 years now. I try to avoid
the union and doing intersection instead (if possible, for shure it is
not the same thing).
1. What you see here
http://trac.osgeo.org/postgis/wiki/UsersWikiExamplesOverlayTables is
very very slow and it will NOT WORK for every case!! For example, if one
Polygon has a inner ring etc. I can not recommand to use it.
2. I have a better code from Regina Obe that works well with smaller
datasets (perhaps up to half a million polygons) i can give to you.
3. Birgit Laggner wrote a function that does the ArcMap Union very good,
but it is not very easy to understand and also not suitable for large
datasets (depends on your Hardware and time you have around 1 million
Polygons).
Contact me if you want to have further informations.....
cheers Andreas
On 03.01.2011 16:05, ΑΠΟΣΤΟΛΟΣ ΛΕΛΕΝΤΖΗΣ wrote:
I try to execute the basic functionalities of GIS (union, sym_difference,
buffer, intersect...) with PostGIS 1.5, but i encounter some problems.
I create a new table to store results of overlay analysis, and i observe
that every entity of new thematic layer is triplicate!!
Specifically, i want to union two layers which include 15 and 3 entities
respectively, and i use the following command:
CREATE TABLE tablename AS
SELECT ST_UNION (a.the_geom, b.the_geom)
FROM table1 AS a, table2 AS b ;
and i note that the above SQL query relate each entity from one layer with
each entity from other, like a cross join!!!
What happened??
Also, i find that the new thematic layer isn't add to the table
"geometry_columns"!!!Why??
Thanks,
Apostolis
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--
Dipl. Geoökologe Andreas Laggner
Institut für Agrarrelevante Klimaforschung (AK) des vTI
Arbeitsgruppe Emissionsinventare
Johann Heinrich von Thünen-Institut (vTI),
Bundesforschungsinstitut für Ländliche Räume, Wald und Fischerei
Institute of Agricultural Climate Research (AK) of the vTI
Johann Heinrich von Thünen-Institute (vTI),
Federal Research Institute for Rural Areas, Forestry and Fisheries
Bundesallee 50
D-38116 Braunschweig
Tel.: (+49) (0)531 596 2636
Fax : (+49) (0)531 596 2645
E-mail: andreas.lagg...@vti.bund.de
Homepage: http://www.vti.bund.de
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