I try to execute the basic functionalities of GIS (union, sym_difference, buffer, intersect...) with PostGIS 1.5, but i encounter some problems. I create a new table to store results of overlay analysis, and i observe that every entity of new thematic layer is triplicate!! Specifically, i want to union two layers which include 15 and 3 entities respectively, and i use the following command: CREATE TABLE tablename AS SELECT ST_UNION (a.the_geom, b.the_geom) FROM table1 AS a, table2 AS b ; and i note that the above SQL query relate each entity from one layer with each entity from other, like a cross join!!! What happened?? Also, i find that the new thematic layer isn't add to the table "geometry_columns"!!!Why??
Thanks, Apostolis
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