Hi Remi/all, I realise I'm digressing slightly from the point, but this seems related. This seems strange to me:
I check the geometry type of a table of what I think are linestrings (roads): ukroads=# SELECT COUNT(*), geometrytype(geom) FROM ukrds GROUP BY geometrytype(geom); 395356 | LINESTRING Great. They are all linestrings. Now I check that none of them are self-intersecting: ukroads=# SELECT COUNT(*), st_issimple(geom) FROM ukrds GROUP BY st_issimple(geom); 395356 | t Ok. That's fine. They are all ok. Now I check that they are all valid: ukroads=# SELECT COUNT(*), st_isvalid(geom) FROM ukrds GROUP BY st_isvalid(geom); 395356 | t So now I buffer them all by 1000 metres wanting to make a polygon out of each line: ukroads=# SELECT COUNT(a.*), geometrytype(a.the_geom) FROM (SELECT st_buffer(geom,1000) as the_geom FROM ukrds) AS a GROUP BY geometrytype(a.the_geom); 392255 | POLYGON 3101 | MULTIPOLYGON However I am returned with 3101 multipolygons (and the rest polygons). Why is that? Should they not all be polygons? Why do some of them become a multipolygon? Thanks James On 25 November 2013 17:25, Rémi Cura <remi.c...@gmail.com> wrote: > You could use the default ending (which is well defined), > then split the resulting with the line like endcap=flat (easy to build, > translate endpoint by radius and -radius in normal direction) > > I still fail to understand why you would need this kind of ending. > > Cheers, > > Rémi C > > > 2013/11/25 Rémi Cura <remi.c...@gmail.com> >> >> Does offset curve gives the same result? >> (seems lile offsetting both side have same behavior) >> >> Maybe you can try several buffers with increasing size? >> (default appears wau before 1000) >> >> Also, can you try to simplify your line : each coordinates uses 15 digits, >> surely you don't need all of this ! >> (simplifying to 8 digits doesn't help). >> >> It seems like a bad design in algorithm ? >> >> Cheers, >> Rémi-C >> >> >> 2013/11/25 James David Smith <james.david.sm...@gmail.com> >>> >>> Hi there, >>> >>> Some code to illustrate my problem: >>> >>> 1) A linestring (SRID: 27700) >>> >>> LINESTRING(555936.152 200920.582000002,555938.312000002 >>> 200908.102000002,555943.112000001 200883.142000001,555953.192000001 >>> 200839.702,555964.471999999 200798.181999998,555974.312 >>> 200764.342000002,555983.912 200744.182000002,555990.554 >>> 200733.721000003,555993.512 200729.062000005,555995.778000002 >>> 200726.756000001,556006.952000001 200715.382000001,556024.232 >>> 200698.822000002,556036.597999999 200687.931,556050.392000001 >>> 200675.782000002,556055.914 200671.265000002,556071.512 >>> 200658.502000002,556094.915000001 200640.537000002,556095.451000001 >>> 200640.152000001,556113.992000001 200628.742000001,556138.472000001 >>> 200616.502000004,556159.112000002 200605.942000002,556180.232000001 >>> 200589.862000002,556207.592 200568.022000002,556217.912000002 >>> 200558.182,556228.472000001 200545.702,556240.472 >>> 200527.702000003,556251.992000001 200509.221999999,556253.237000001 >>> 200506.732000003,556258.952 200495.302000001,556268.000000001 >>> 200478.000000002,556279.592 200458.582000002,556300 >>> 200431.000000002,556351.000000002 200364,556349.253 200366.234000001) >>> >>> 2) Now I buffer it: >>> >>> SELECT ST_Buffer( >>> ST_GeomFromText( >>> 'LINESTRING(555936.152 200920.582000002,555938.312000002 >>> 200908.102000002,555943.112000001 200883.142000001,555953.192000001 >>> 200839.702,555964.471999999 200798.181999998,555974.312 >>> 200764.342000002,555983.912 200744.182000002,555990.554 >>> 200733.721000003,555993.512 200729.062000005,555995.778000002 >>> 200726.756000001,556006.952000001 200715.382000001,556024.232 >>> 200698.822000002,556036.597999999 200687.931,556050.392000001 >>> 200675.782000002,556055.914 200671.265000002,556071.512 >>> 200658.502000002,556094.915000001 200640.537000002,556095.451000001 >>> 200640.152000001,556113.992000001 200628.742000001,556138.472000001 >>> 200616.502000004,556159.112000002 200605.942000002,556180.232000001 >>> 200589.862000002,556207.592 200568.022000002,556217.912000002 >>> 200558.182,556228.472000001 200545.702,556240.472 >>> 200527.702000003,556251.992000001 200509.221999999,556253.237000001 >>> 200506.732000003,556258.952 200495.302000001,556268.000000001 >>> 200478.000000002,556279.592 200458.582000002,556300 >>> 200431.000000002,556351.000000002 200364,556349.253 >>> 200366.234000001)'), 1000, 'endcap=flat join=round'); >>> >>> 3) The result is attached as a jpg (line thickness increased to aid >>> viewing). >>> >>> Any ideas please? This is related to an ongoing discussion I was >>> having with Remi a while ago. Basically I'm buffering loads of road >>> centrelines to create polygons. But when I do it, a small number end >>> up with really strange buffers like this attached example. I'm at a >>> loss as to why. >>> >>> Thanks >>> >>> James >>> >>> _______________________________________________ >>> postgis-users mailing list >>> postgis-users@lists.osgeo.org >>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users >> >> > > > _______________________________________________ > postgis-users mailing list > postgis-users@lists.osgeo.org > http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users