[postgis-users] Odd st_buffer behaviour

[James David Smith]

Apologies. I think I've just answered this myself. It's the
'endcap=flat join=round' bit isn't it. It's not doing what I am
expecting it to do. Though I'm not quite sure how to fix it yet.

On 25 November 2013 17:04, James David Smith
<james.david.smith at gmail.com> wrote:
> 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