[postgis-users] Odd st_buffer behaviour

[James David Smith]

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
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