[postgis-users] problem creating and exporting a polygon with holeusing pgsql2shp

[Obe, Regina]

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: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Ghislain Geniaux
Sent: Wednesday, March 12, 2008 6:29 AM
To: postgis-users at postgis.refractions.net
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.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20080312/d5262422/attachment.html>