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

[Obe, Regina]

Could be your export routine then.  I don't think ' is legal.  Need to use double-quotes (or at least on windows) for the SQL. You can also leave out the .shp although I guess its harmless.
 
Try
pgsql2shp -f ext conversion "select * from matable2"
 
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 8:47 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] problem creating and exporting a polygon withholeusing pgsql2shp


Thanks for your answer Regina, 


but ST_Difference does not solve the problem :  
ST_Difference and ST_Symdifference give the same polygon but the export by pgsql2shp failled.


conversion=# create table temp as 
conversion-# select  r.gid, ST_Difference(r.the_geom,i.the_geom) as the_geom from recouv as r, recouv as i where i.gid=2 and r.gid=1;
SELECT
conversion=# ALTER TABLE temp ADD PRIMARY KEY (gID);
NOTICE:  ALTER TABLE / ADD PRIMARY KEY will create implicit index "temp_pkey" for table "temp"
ALTER TABLE
conversion=# SELECT geometrytype(the_geom), Nrings(the_geom), area2d(the_geom) from temp;
 geometrytype | nrings |      area2d      
--------------+--------+------------------
 POLYGON      |      2 | 2177713.02017212
(1 row)




conversion=# create table temp as 
conversion-# select  r.gid, ST_symDifference(r.the_geom,i.the_geom) as the_geom from recouv as r, recouv as i where i.gid=2 and r.gid=1;
SELECT
conversion=# ALTER TABLE temp ADD PRIMARY KEY (gID);
NOTICE:  ALTER TABLE / ADD PRIMARY KEY will create implicit index "temp_pkey" for table "temp"
ALTER TABLE
conversion=# SELECT geometrytype(the_geom), Nrings(the_geom), area2d(the_geom) from temp;
 geometrytype | nrings |      area2d      
--------------+--------+------------------
 POLYGON      |      2 | 2177713.02017212
(1 row)


The two polygons are goods and well displayed in QGIS but exported shapefiles are empty or corrupted.






Le 12 mars 08 à 12:51, Obe, Regina a écrit :


	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



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