[postgis-devel] [PostGIS] #1973: st_concavehull() returns sometimes empty geometry collection

[PostGIS]

#1973: st_concavehull() returns sometimes empty geometry collection
-----------------------------------------------------------+----------------
 Reporter:  tolke                                          |       Owner:  pramsey      
     Type:  defect                                         |      Status:  new          
 Priority:  medium                                         |   Milestone:  PostGIS 2.1.0
Component:  postgis                                        |     Version:  2.0.x        
 Keywords:  concave hull st_concavehull points_as_polygon  |  
-----------------------------------------------------------+----------------
 Executing the test query below shows that st_concavehull() returns
 sometimes an empty (and thus incorrect) polygon. For demonstration, the
 st_extent() is returned which is also empty in this case.

 ---

 DROP TABLE IF EXISTS temp;

 CREATE TEMPORARY TABLE temp AS SELECT
 ST_MultipointFromText('MULTIPOINT(-0.1116977 51.4968911,-0.1144059
 51.4998103,-0.1145644 51.4997663,-0.1117435 51.4969843,-0.11243
 51.4985909,-0.1126424 51.4983149,-0.1095458 51.4983205,-0.1103099
 51.4982817,-0.1145062 51.4998847,-0.1124628 51.4988542,-0.1111448
 51.5002042,-0.1118638 51.4994708,-0.1125271 51.4997354,-0.1117703
 51.5003853,-0.1104727 51.5016664,-0.1106169 51.4982728,-0.1101081
 51.4999338,-0.1108346 51.5005757,-0.110639 51.501701,-0.1106653
 51.5016968,-0.109791 51.4998492,-0.1129582 51.5000033,-0.1119142
 51.4986353,-0.112254 51.4983439,-0.1092107 51.4983322,-0.1107878
 51.4983057,-0.1108303 51.4982275,-0.1104942 51.5015336,-0.1107248
 51.5001007,-0.110516 51.5008405,-0.1105331 51.5011704,-0.1118708
 51.5002901,-0.1115661 51.5001851,-0.111476 51.5002775,-0.1141657
 51.4996968,-0.1128279 51.4981607,-0.1107347 51.5016685,-0.1106417
 51.5008641,-0.110439 51.5017415,-0.1106903 51.5016875)') AS geom;

 SELECT st_extent(geom) FROM temp; -- ok

 SELECT st_extent(st_concavehull(geom,.9)) FROM temp; -- empty(!)

 After removing the lass three coordinates and executing the query above
 again, a correct polygon is computed, resulting in an non-empty
 st_extent().

 Btw, points_as_polygon() returns non-empty results.

 SELECT postgis_full_version(): "POSTGIS="2.0.1 r9979"
 GEOS="3.3.5-CAPI-1.7.5" PROJ="Rel. 4.7.1, 23 September 2009"
 LIBXML="2.6.26""

 Please fix this defect :-)

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/1973>
PostGIS <http://trac.osgeo.org/postgis/>
The PostGIS Trac is used for bug, enhancement & task tracking, a user and developer wiki, and a view into the subversion code repository of PostGIS project.