[postgis-users] problem clipping a feature

[Kevin Neufeld]

Hi Andre,

 From your query, it looks like your resultant table, pg_voting_areas, is going to have two geometry columns: 
'intersection_geom' (an alias provided after ST_Intersection) and 'the_geom' (obtained from v.*, bc_voting_areas.the_geom).

You mentioned that you tried to add 'the_geom' to the geometry_columns table to help out QuantumGIS.  You may want to do 
the same for 'intersection_geom' so you can visualize the clipped data, not just the voting areas.

I'm not very familiar with QuantumGIS, but it sounds like it gets confused with tables that have more than one geometry 
column - you mentioned Quantum recognized this table as having two geometry columns with the same name ... as I pointed 
out, this is not the case.  Try selecting the 'intersection_geom' from pg_voting_areas into a separate table and try to 
visualize that.

Hope this helps,
Kevin

Andre Oliveira wrote:
> Hi Everybody,
> 
> I was trying to use the ST_Intersection function to overlay (clip) a 
> poygon by another, with the following code from the workshop at foss4g2007:
> 
> CREATE TABLE pg_voting_areas AS
> SELECT
>   ST_Intersection(v.the_geom, m.the_geom) AS intersection_geom,
>   ST_Area(v.the_geom) AS va_area,
>   v.*,
>   m.name <http://m.name>
> FROM
>   bc_voting_areas v,
>   bc_municipality m
> WHERE
>   ST_Intersects(v.the_geom, m.the_geom) AND
>   m.name <http://m.name> = 'PRINCE GEORGE';
>  
> After this operation I restarted pgAdminIII, generated an index for 
> pg_voting_areas, run vacuum analyze, and defined GID as the primary key 
> for pg_voting_areas.
> 
> But when I try to connect with QuantumGIS to visualize the data, two 
> datasets where recognized with the same name, one bearing the expected 
> result, but the other a display with more objects than expected for a clip.
> 
> Then I tried to inserted the dataset's line into table geometry_columns: 
> 
> INSERT INTO geometry_columns (f_table_catalog, f_table_schema, 
> f_table_name,f_geometry_column, coord_dimension, srid, type)
> 
> SELECT '', 'public', 'pg_voting_areas', 'the_geom',
> ST_CoordDim(the_geom),
> ST_SRID(the_geom),
> GeometryType(the_geom)
> 
> FROM public.pg_voting_areas LIMIT 1;
> 
> After I did this, one of the datasets recognized by QuantumGIS 
> disappeared, but it was the undesired one that survived. Also, I noticed 
> that some of the measurements in the workshop are not totally equal to 
> the ones I obtain (for instance, when I query the clipped poygon's area 
> and the clipping municipality's area, they are not exactly equal - 
> 326579103.825073 versus 326579103.825928). 
>  
> I am using the last versions of postgre, postgis and quantumgis.
> When I installled postgre I chose locale by default and created the 
> postgis database manually, bearing UTF8 encoding.
> 
> I would really appreciate a hand here.
> Thanks
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users