[postgis-tickets] [PostGIS] #2864: ST_Envelope does not contain all points of invalid geometry

[PostGIS]

#2864: ST_Envelope does not contain all points of invalid geometry
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 Reporter:  dbaston  |       Owner:  pramsey
     Type:  defect   |      Status:  new    
 Priority:  high     |   Milestone:         
Component:  postgis  |     Version:  2.1.x  
 Keywords:           |  
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 I'm calling this a "defect" although I understand that is open to
 debate...

 When called on an invalid polygon whose interior rings go outside of an
 exterior ring, ST_Envelope can return a box that does not contain all
 points that make up the geometry.  Simple example:

 {{{ SELECT ST_AsText(ST_Envelope('POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0),
 (1 1, 1 2, 2 2, 2 1, 1 -20, 1 1))'::geometry)) -- returns POLYGON((0 0,0
 10,10 10,10 0,0 0)) }}}

 I can see that the concept of an "envelope" becomes fuzzy for a
 meaningless polygon, but a downstream effect is that ST_YMin gives a
 result that is not the minimum y coordinate:

 {{{ SELECT ST_YMin('POLYGON ((0 0, 10 0, 10 10, 0 10, 0 0), (1 1, 1 2, 2
 2, 2 1, 1 -20, 1 1))'::geometry) -- returns 0 }}}

 I didn't look into how PostGIS is determining an envelope (GEOS?) but the
 behaviour of PostGIS in this case _is_ consistent with JTS.

 (My use case here?  Finding all geometries that had coordinates mistakenly
 altered from 40 to 4, 30 to 3, etc.)

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Ticket URL: <http://trac.osgeo.org/postgis/ticket/2864>
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