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

[Ghislain Geniaux]

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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------------------------------------------
Geniaux Ghislain
INRA SAD Ecodéveloppement
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