[postgis-users] problem clipping a feature

[Andre Oliveira]

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
FROM
  bc_voting_areas v,
  bc_municipality m
WHERE
  ST_Intersects(v.the_geom, m.the_geom) AND
  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
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