[postgis-users] Union of 2 polygon shape files

[RAVI KUMAR]

Hi Havard,
what are equal opperation of Overlay in Post GIS.
Ravi Kumar
Havard Tveite <havard.tveite at umb.no> wrote: Dear Ravi,

You should specify what you expect the Union operation
to do.
The PostGIS (ST_)Union is not the same as an overlay (if
that is what you want to achieve).
The union operation joins two geometries or a set of
geometries (the aggregation version) together to form a
new geometry.

Håvard


ST_Union(geometry, geometry)
     Returns a geometry that represents the point set union of the 
Geometries.
     Performed by the GEOS module
     Do not call with a GeometryCollection as an argument
     NOTE: this is renamed from "union" because union is an SQL reserved 
word
     OGC SPEC s2.1.1.3
ST_Union(geometry set)
     Returns a geometry that represents the point set union of this all 
Geometries in given set.
     Performed by the GEOS module
     Do not call with a GeometryCollection in the argument set
     Not explicitly defined in OGC SPEC


RAVI KUMAR wrote:
> Hi,
> I find that Union of 2 polygon shape files is not what is expected from 
> other FOSS GIS like GRASS etc. I can send two small shape files with a 
> few polygons and the anticipated result. Pl let me know to whom I should 
> mail it for help, such that UNION with PostGIS can be used after 
> necessary modifications.
> 
> This will clear a roadblock for me in a exercise on Mineral 
> prognastication using FOSS GIS.
> Ravi Kumar
> 
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-- 
--
Håvard Tveite
Department of Mathematical Sciences and Technology
Norwegian University of Life Sciences


       
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