[geos-devel] [GEOS] #725: GEOS allows relate ops on GeometryCollection, while JTS and PostGIS do not

[GEOS]

#725: GEOS allows relate ops on GeometryCollection, while JTS and PostGIS do not
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 Reporter:  mwtoews     |       Owner:  geos-devel@…              
     Type:  task        |      Status:  new                       
 Priority:  minor       |   Milestone:  GEOS Future               
Component:  Default     |     Version:  svn-trunk                 
 Severity:  Unassigned  |    Keywords:                            
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 GEOS currently allows relate ops on GEOMETRYCOLLECTION geometries, e.g.
 with Shapely:
 {{{
 from shapely.wkt import loads
 g1 = loads('GEOMETRYCOLLECTION(POINT(1 2),POINT(3 4))')
 g2 = loads('GEOMETRYCOLLECTION(POINT(3 4),POINT(1 2))')
 print(g1.equals(g2))  # True
 print(g1.relate(g2))  # 0FFFFFFF2
 }}}
 or with R
 {{{
 library(rgeos)
 g1 <- readWKT("GEOMETRYCOLLECTION(POINT(1 2),LINESTRING(3 4, 5 6))")
 g2 <- readWKT("GEOMETRYCOLLECTION(LINESTRING(3 4, 5 6),POINT(1 2))")
 g3 <- readWKT("GEOMETRYCOLLECTION(LINESTRING(3 4, 5 6),POINT(1 1))")
 gEquals(g1, g2)  # [1] TRUE
 gRelate(g1, g2)  # [1] "1FFF0FFF2"
 gRelate(g1, g3)  # [1] "1F0F0F0F2"
 }}}

 PostGIS and JTS do not allow these operations, see email threads:
  * http://lists.osgeo.org/pipermail/postgis-devel/2015-April/024827.html
  * http://lists.osgeo.org/pipermail/postgis-devel/2015-April/024829.html

 In particular, the rationale for the restriction in JTS from Martin Davis:
 > It's because GeometryCollection have no inherent topology.  In other
 words,
 > geometry components can overlap.  To compute relationships with a
 geometry
 > with overlapping components the components have to be merged to form a
 > topologically valid arrangement.  This is more complex and more
 > computationally expensive than the rest of the relate algorithm.  So
 > basically it just didn't get done.  (It would also introduce potential
 > robustness issues, which I was hoping to avoid in the relate code.

 > A few relationships (eg intersects and disjoint) could be computed more
 > easily, so they could have special case code added for them more easily.

 This task is to assess the situation for GEOS, whether a similar
 restriction should be set for relate ops, or any other changes. Note that
 I haven't found a case that returns a bad result, but I haven't looked
 hard.

-- 
Ticket URL: <http://trac.osgeo.org/geos/ticket/725>
GEOS <http://trac.osgeo.org/geos>
GEOS (Geometry Engine - Open Source) is a C++ port of the Java Topology Suite (JTS).