In addition to Nicolas' suggestion, one thing that can work, for example when the goal is to "make holes" with one theme in another, is to st_union the two themes, then re-attribute, then delete the unwanted polygons. This has worked for me in relatively small datasets but I've never truly stress tested it.
2011/9/7 Nicolas Ribot <nicolas.ri...@gmail.com> > > Hello Group, > > > > About st_difference I've seen several topics but unfortunately with none > of them I could solve my problem. > > > > What I want to do is subtract one layer from the other. I could simplify > this down to the following; > > > > Tbl_a contains 1 record > > "POLYGON(((168119.443682473 451093.811197312,197555.469699649 > 451093.811197312,197555.469699649 437539.54805452,168119.443682473 > 437539.54805452,168119.443682473 437539.54805452,168119.443682473 > 451093.811197312)))" > > > > Tbl_b contains 2 records of which both intersects with the object in > tbl_a > > > > "POLYGON(((171424.484910418 453680.34544058,171424.484910418 > 435697.56759188,174568.140889894 435697.56759188,174568.140889894 > 453680.34544058,174568.140889894 453680.34544058,171424.484910418 > 453680.34544058)))" > > > > "POLYGON(((184904.90800885 453627.063135843,184904.90800885 > 435644.285287143,188048.563988327 435644.285287143,188048.563988327 > 453627.063135843,188048.563988327 453627.063135843,184904.90800885 > 453627.063135843)))" > > > > The result I'm looking for is the object of tbl_a abstracted with both > objects from tbl_b, since they are both intersecting. In this example the > result should be 3 squares since it is cut twice by tbl_b. > > > > > > Does anyone know a solution for this? > > > > Many thanks in advance, > > > > Ge > > Hi Ge, > > You may want to union objects in table B to create a single polygon, > then perfom the difference: > > > select st_difference(a.geom, b.geom) > from > (select 'POLYGON((168119.443682473 451093.811197312,197555.469699649 > 451093.811197312,197555.469699649 437539.54805452,168119.443682473 > 437539.54805452,168119.443682473 437539.54805452,168119.443682473 > 451093.811197312))'::geometry as geom) as a, > ( > select st_union(geom) as geom > from ( > select 'POLYGON((171424.484910418 453680.34544058,171424.484910418 > 435697.56759188,174568.140889894 435697.56759188,174568.140889894 > 453680.34544058,174568.140889894 453680.34544058,171424.484910418 > 453680.34544058))'::geometry as geom > UNION > select 'POLYGON((184904.90800885 453627.063135843,184904.90800885 > 435644.285287143,188048.563988327 435644.285287143,188048.563988327 > 453627.063135843,188048.563988327 453627.063135843,184904.90800885 > 453627.063135843))'::geometry as geom > ) as foo > ) as b; > > (see attached pictures) > > Nicolas > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > > -- Chris Hermansen *Vice President* TECO Natural Resource Group Limited 301 · 958 West 8th Avenue Vancouver BC CANADA · V5Z 1E5 Tel +1.604.714.2878 · Cel +1.778.840.4625
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