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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