[postgis-tickets] [PostGIS] #2181: ST_Buffer returning multipolygon with linestring input

[PostGIS]

#2181: ST_Buffer returning multipolygon with linestring input
----------------------------------------------+-----------------------------
 Reporter:  mcastrog                          |       Owner:  pramsey      
     Type:  defect                            |      Status:  new          
 Priority:  medium                            |   Milestone:  PostGIS 1.5.9
Component:  postgis                           |     Version:  1.5.X        
 Keywords:  Buffer, linestring, multipolygon  |  
----------------------------------------------+-----------------------------
 Hi,
 I got the standard North Carolina dataset
 http://grass.osgeo.org/sampledata/north_carolina/nc_shape.tar.gz and
 imported ''streams'' shape layer into postgresql through shp2psql shell
 command.[[BR]]
 In the resulting table the geometry field contains only LINESTRING
 elements.

 By running the following query:


 {{{
 SELECT
 sub.gid, GeometryType(sub.buffer) AS buffer_GeometryType,
 GeometryType(sub.the_geom) AS stream_GeometryType
 FROM (
         SELECT gid, the_geom, st_buffer (the_geom, 100) AS buffer
         FROM streams
 ) AS sub
 WHERE
 GeometryType(sub.buffer)  <> 'POLYGON';
 }}}


 I get this output:


 {{{
  gid  | buffer_geometrytype | stream_geometrytype
 ------+---------------------+---------------------
   674 | MULTIPOLYGON        | LINESTRING
   810 | MULTIPOLYGON        | LINESTRING
   811 | MULTIPOLYGON        | LINESTRING
  1564 | MULTIPOLYGON        | LINESTRING
  1571 | MULTIPOLYGON        | LINESTRING
  1667 | MULTIPOLYGON        | LINESTRING
  2764 | MULTIPOLYGON        | LINESTRING
  2848 | MULTIPOLYGON        | LINESTRING
  5203 | MULTIPOLYGON        | LINESTRING
  5250 | MULTIPOLYGON        | LINESTRING
  8369 | MULTIPOLYGON        | LINESTRING

 }}}

 So, 11 out of 8554 records are MULTIPOLYGONs not POLYGONs as expected.

 To be more accurate, I isolated the record having gid=674


 {{{
 SELECT gid, AsText(the_geom)
 FROM streams
 WHERE gid=674;
 }}}


 the related output is:


 {{{
  674 | LINESTRING(640770.332537465 216785.425146015,640770.356311913
 216785.560172686,640770.475488952 216786.226162019,640770.647396495
 216787.800152365,640770.658978918 216789.268376902,640770.447752096
 216792.399898366,640769.98018796 216795.624688816,640769.110287021
 216798.527305023,640767.648768098 216801.891389348,640766.02448565
 216804.805892778,640764.74005588 216807.134569434,640763.337363474
 216809.560477487,640761.650901703 216812.246990061,640759.955600712
 216815.327305019,640758.482499366 216817.868732504,640757.383693167
 216819.548488661,640756.238862077 216821.226720821,640754.01503683
 216824.434442434,640751.366929134 216828.435255234,640749.204368809
 216832.450088865,640747.25516891 216836.227787618,640746.014020829
 216838.801219168,640745.393751588 216840.360274284,640744.315976633
 216842.854457673,640743.400050801 216844.65704848,640742.582575565
 216846.277673322,640741.52491745 216848.478638524,640740.503835408
 216850.705816576,640739.787553975 216852.270967708,640739.053289307
 216853.851968469,640738.173025147 216855.546355058,640737.55488951
 216856.568961104,640736.817272035 216857.822910812,640735.910795021
 216859.35697228,640734.77510795 216861.236982439,640733.832969266
 216862.938074642,640732.814325629 216865.039674844,640731.225095251
 216869.012141189,640729.979984761 216871.879095724,640729.445974092
 216873.02148841,640729.002794006 216873.679857725,640728.952197105
 216873.745389857,640728.676962154 216874.089814544)

 }}}

 All of you can see that the above record represents a regular LINESTRING
 geometry.
 Finally I exported the  gid=674 stream and the related buffer, you can see
 large and small zoom at this links:

 https://docs.google.com/file/d/0B0MkYF-Ljs34c1owdkc1U0Q4OFk/edit
 https://docs.google.com/file/d/0B0MkYF-Ljs34MlNkTkJwc0dEbG8/edit

 The behaviour is not regular, I suppose.

 About postis version


 {{{
 SELECT postgis_full_version();
 }}}


 returns

 POSTGIS="1.5.2" GEOS="3.2.2-CAPI-1.6.2" PROJ="Rel. 4.7.1, 23 September
 2009" LIBXML="2.7.8" USE_STAT

 Best wishes.[[BR]]
 Marco

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/2181>
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