[postgis-users] Odd st_buffer behaviour

[James David Smith]

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.cura at 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.cura at 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.smith at 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 at lists.osgeo.org
>>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>>
>
>
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