Hi Havard,<br>what are equal opperation of <span style="background-color: rgb(255, 255, 191);">Overlay in Post GIS</span>.<br>Ravi Kumar<br><b><i>Havard Tveite <havard.tveite@umb.no></i></b> wrote:<blockquote class="replbq" style="border-left: 2px solid rgb(16, 16, 255); margin-left: 5px; padding-left: 5px;"> Dear Ravi,<br><br>You should specify what you expect the Union operation<br>to do.<br>The PostGIS (ST_)Union is not the same as an overlay (if<br>that is what you want to achieve).<br>The union operation joins two geometries or a set of<br>geometries (the aggregation version) together to form a<br>new geometry.<br><br>Håvard<br><br><br>ST_Union(geometry, geometry)<br> Returns a geometry that represents the point set union of the <br>Geometries.<br> Performed by the GEOS module<br> Do not call with a GeometryCollection as an argument<br> NOTE: this is renamed from "union" because union is an SQL reserved <br>word<br> OGC SPEC
s2.1.1.3<br>ST_Union(geometry set)<br> Returns a geometry that represents the point set union of this all <br>Geometries in given set.<br> Performed by the GEOS module<br> Do not call with a GeometryCollection in the argument set<br> Not explicitly defined in OGC SPEC<br><br><br>RAVI KUMAR wrote:<br>> Hi,<br>> I find that Union of 2 polygon shape files is not what is expected from <br>> other FOSS GIS like GRASS etc. I can send two small shape files with a <br>> few polygons and the anticipated result. Pl let me know to whom I should <br>> mail it for help, such that UNION with PostGIS can be used after <br>> necessary modifications.<br>> <br>> This will clear a roadblock for me in a exercise on Mineral <br>> prognastication using FOSS GIS.<br>> Ravi Kumar<br>> <br>> ------------------------------------------------------------------------<br>> Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try
<br>> it now. <br>> <http: us.rd.yahoo.com="" evt="51733/*http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ"><br>> ><br>> <br><br>-- <br>--<br>Håvard Tveite<br>Department of Mathematical Sciences and Technology<br>Norwegian University of Life Sciences<br></http:></blockquote><br><p> 
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