I'm guessing your result may be a Geometrycollection which ESRI shape has no clue what to do with. If I understand you correctly, I think you really should be using ST_Difference instead of ST_SymDifference. So ST_Difference(polygon2, polygon1) - will give you that portion of polygon2 which does not intersect with polygon1 which would give you a polygon with polygon1 removed (a doughnut so to speak). The ST_Symdifference will give you that portion of polygon2 not in polygon1 + polygon1 not in polygon2 = the holed polygon you want + an empty geometry collection -> a geometry collection if taken strictly. I'm not sure if PostGIS will throw out the second part (the empty collection) - and at anyrate is a waste of processing power to even compute it. Hope that helps, Regina
________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Ghislain Geniaux Sent: Wednesday, March 12, 2008 6:29 AM To: [email protected] Subject: [postgis-users] problem creating and exporting a polygon with holeusing pgsql2shp Using a postgis table with Two polygons, in which polygon 1 entirely contained in polygon 2, I've create a new polygon with a hole using : ------ create table matable2 as select r.gid, area(symdifference(r.the_geom,i.the_geom)) as area,(symdifference(r.the_geom,i.the_geom)) as the_geom from matable as r, matable as i where i.gid=2 and r.gid=1; ALTER TABLE matable2 ADD PRIMARY KEY (GID); \q pgsql2shp -f ext.shp conversion 'select * from matable2' ----- Area is correct, display of the polygon in QGIS is also correct, but the export by pgsl2shp produce an empty shapefile. How to deal with this problem? ------------------------------------------ Geniaux Ghislain ----------------------------------------- The substance of this message, including any attachments, may be confidential, legally privileged and/or exempt from disclosure pursuant to Massachusetts law. It is intended solely for the addressee. If you received this in error, please contact the sender and delete the material from any computer.
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