Steve, Thanks, you are of course right. I hadn't thought of that. Makes sense.
James On 25 November 2013 17:58, Stephen Woodbridge <wood...@swoodbridge.com> wrote: > This is because lines shaped like say an Omega character, will create an > island inside the character at a certain offset and this can not be > represented as a simple polygon. Well maybe it can but the algorithm > probably does not try to reduce them to their most simple features. While > the lines may not intersect, the offset lines may intersect and it is a > function of how these are dealt with. > > -Steve > > > On 11/25/2013 12:49 PM, James David Smith wrote: >> >> 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 >> > > _______________________________________________ > 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