[QGIS-trac] Re: [Quantum GIS] #2921: Avoid intersection of new polygons.. IS BROKEN

[Quantum GIS]

#2921: Avoid intersection of new polygons.. IS BROKEN
-----------------------------------------------------+----------------------
        Reporter:  pinux                             |         Owner:  mhugent               
            Type:  bug                               |        Status:  reopened              
        Priority:  major: does not work as expected  |     Milestone:  Version 1.7.0         
       Component:  Digitising                        |       Version:  Trunk                 
      Resolution:                                    |      Keywords:  intersection, polygons
Platform_version:                                    |      Platform:  All                   
        Must_fix:  Yes                               |   Status_info:  0                     
-----------------------------------------------------+----------------------
Changes (by mayeulk):

  * priority:  critical: causes crash or data corruption => major: does not
               work as expected
  * platform:  Windows => All
  * version:  1.5.0 => Trunk


Comment:

 Hi,

 I can confirm most of the above, with some comments which might help
 debugging.

 You can try this query:


 {{{
 select *, st_area(geom), st_isvalid(geom)  from (select a.gid + b.gid *
 100 AS gid, a.gid AS a_gid, b.gid AS b_gid,
 st_setsrid((st_intersection(a.the_geom, b.the_geom)), 4326) as geom
 FROM av_intersection a, av_intersection b
   WHERE a.gid < b.gid AND st_overlaps(b.the_geom, a.the_geom)) as
 the_intersects;
 }}}

 You'll see that 2 thirds of the intersections have a null area. For the
 other ones, their area is always (on my machine) 0.00048828125 and I
 believe it is a Postgis/geos error, see below:



 {{{
 WITH av_intersect_911 as (
 select 91104 AS gid,
 st_setsrid(ST_GeometryN((st_intersection(a.the_geom, b.the_geom)),4),
 4326) as geom
 FROM av_intersection a, av_intersection b
   WHERE a.gid=9 and b.gid=11
 limit 1)

 SELECT gid, geom, GeometryType (av_intersect_911.geom),
 st_area(av_intersect_911.geom), ST_AsEWKT (av_intersect_911.geom)

 FROM av_intersect_911;

 }}}

 The above query shows a polygon which is (part of) the intersection of
 polygons 9 and 11. It's area is  0.00048828125. This polygon has this EWKT
 definition:


 {{{
 "SRID=4326;POLYGON((
 1517034.97851789 5634807.73109361,
 1517034.97851789 5634807.73109361,
 1517034.11662417 5634808.29056848,
 1517034.97851789 5634807.73109361))"
 }}}

 Since 3 of its four points are identical, it is trivial to say that this
 polygon is flat and its area is zero, hence the area computation is false.

 I am not 100% sure that there are "real" intersections, as several of the
 intersections are not polygons but lines at the boundary of 2 polygons.
 Still, I agree that the topology is incorrect: the topology is broken.

 Still, this does not corrupt existing data nor create invalid geometry:
 they can be displayed in qgis and the following is true in postgis:

 {{{
 select st_isvalid(the_geom) from av_intersection;
 }}}

 Given this postgis bug, I tried to reproduce this with a shapefile (to
 make sure that we are hunting a QGIS bug, not a postgis bug).

 Below an ASCCI art explanation of what I found, please see also attached
 screenshot and attached shapefile.

 {{{

 You have A:
 ____
     |
 A   |
 -----

 You add B:

 ____________
     |B     /
 A   |     /
 ----|----/

 Everything is OK until you add C:

    ____
   / C  \
 _/______\___
     |B     /
 A   |     /
 ----|----/

 You expect C to be like this:
    ____
   / C  \
  /______\

 The topological error that I can confirm is the following: C is like this:
    ____
   / C  \
  /______\
     |
     |

 My conclusion:
 When you add a polygon C where two polygons A and B are
 touching (or are supposed to), then C will have a leg going
 in between A and B. It is not clear whether A and B are
 strictly contiguous or if there is an (unwanted) hole between
 them.



 }}}

-- 
Ticket URL: <http://trac.osgeo.org/qgis/ticket/2921#comment:10>
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