[geos-devel] Re: [GEOS] #253: Unexpected "Self-intersection at or near point"

[GEOS]

#253: Unexpected "Self-intersection at or near point"
-------------------------------+--------------------------------------------
 Reporter:  wvereeck           |        Owner:  geos-devel at lists.osgeo.org
     Type:  defect             |       Status:  new                       
 Priority:  minor              |    Milestone:  3.1.1                     
Component:  Core               |      Version:  3.1.0                     
 Severity:  Unassigned         |   Resolution:                            
 Keywords:  self-intersection  |  
-------------------------------+--------------------------------------------
Old description:

> Hi All,
>
> I'm very new to GEOS (< 1 day), but I'm puzzled by what I think is a
> strange bug.
> Being particularly interested in the GEOSUnionCascaded() function to
> merge large sets (>50,000) of overlapping polygons, altering my dataset
> manually to avoid this bug(?) is not an option.
>
> I have narrowed down the problem to the following multipolygon, which
> represents a rectangle inside an octagon (see
> http://homes.esat.kuleuven.be/~wvereeck/rect_inside_oct.png)
>
> MULTIPOLYGON(((12.26 18.46, 11.76 18.46, 11.54 18.24, 11.54 17.34, 11.76
> 17.12, 12.26 17.12, 12.48 17.34, 12.48 18.24, 12.26 18.46)),((12.37
> 18.35, 11.65 18.35, 11.65 17.23, 12.37 17.23, 12.37 18.35)))
>
> I am using the c-interface. Running the GEOSisValid() test on this
> polygon set results in:
>    NOTICE: Self-intersection at or near point 12.37 18.35
> Also, GEOSUnionCascaded() accordingly fails with:
>    ERROR: TopologyException: found non-noded intersection between 11.65
> etc...
>
> However, when I slightly offset the x-position (+1e-8) of the two top
> vertices of the rectangle, everything works like a charm (i.e.
> GEOSisValid returns 'true' and GEOSUnionCascaded produces the correct
> output polygon):
>
> MULTIPOLYGON(((12.26 18.46, 11.76 18.46, 11.54 18.24, 11.54 17.34, 11.76
> 17.12, 12.26 17.12, 12.48 17.34, 12.48 18.24, 12.26 18.46)),
> ((12.37000001 18.35, 11.65000001 18.35, 11.65 17.23, 12.37 17.23,
> 12.37000001 18.35)))
>
> I apologize if I'm too much in a hurry here and should start to learn
> more about geos before submitting a complaint :) At least I hope this is
> the right place for this problem.
>
> Kind Regards,
> Wim Vereecken
>
> FWIW, I'm using geos-3.1.0.tar.bz2 (2009/03/17), libgeos-3.1.0 and Ubuntu
> 8.04.2 (x86-64) with a 2.6.24-23-generic kernel.

New description:

 Hi All,

 I'm very new to GEOS (< 1 day), but I'm puzzled by what I think is a
 strange bug.

 Being particularly interested in the GEOSUnionCascaded() function to merge
 large sets (>50,000) of overlapping polygons, altering my dataset manually
 to avoid this bug(?) is not an option.

 I have narrowed down the problem to the following multipolygon, which
 represents a rectangle inside an octagon (see
 http://homes.esat.kuleuven.be/~wvereeck/rect_inside_oct.png)

 {{{
 MULTIPOLYGON(((12.26 18.46, 11.76 18.46, 11.54 18.24, 11.54 17.34, 11.76
 17.12, 12.26 17.12, 12.48 17.34, 12.48 18.24, 12.26 18.46)),((12.37 18.35,
 11.65 18.35, 11.65 17.23, 12.37 17.23, 12.37 18.35)))
 }}}

 I am using the c-interface. Running the GEOSisValid() test on this polygon
 set results in:

 {{{
    NOTICE: Self-intersection at or near point 12.37 18.35
 }}}

 Also, GEOSUnionCascaded() accordingly fails with:

 {{{
    ERROR: TopologyException: found non-noded intersection between 11.65
 }}}

 etc.

 However, when I slightly offset the x-position (+1e-8) of the two top
 vertices of the rectangle, everything works like a charm (i.e. GEOSisValid
 returns 'true' and GEOSUnionCascaded produces the correct output polygon):

 {{{
 MULTIPOLYGON(((12.26 18.46, 11.76 18.46, 11.54 18.24, 11.54 17.34, 11.76
 17.12, 12.26 17.12, 12.48 17.34, 12.48 18.24, 12.26 18.46)), ((12.37000001
 18.35, 11.65000001 18.35, 11.65 17.23, 12.37 17.23, 12.37000001 18.35)))
 }}}

 I apologize if I'm too much in a hurry here and should start to learn more
 about geos before submitting a complaint :) At least I hope this is the
 right place for this problem.

 Kind Regards,
 Wim Vereecken

 FWIW, I'm using geos-3.1.0.tar.bz2 (2009/03/17), libgeos-3.1.0 and Ubuntu
 8.04.2 (x86-64) with a 2.6.24-23-generic kernel.

-- 
Ticket URL: <http://trac.osgeo.org/geos/ticket/253#comment:1>
GEOS <http://geos.refractions.net/>
GEOS (Geometry Engine - Open Source) is a C++ port of the Java Topology Suite (JTS).