[postgis-users] st_intersection error

[Düster Horst]

Niklas
 
Thank you very much for the offered solution. In my case it solves my
problem with a very tiny buffer distance below the accuracy of my data. 
 
Regrads
Horst
 

------------------------------------------------ 

Dr. Horst Düster 
Stv. Amtschef / GIS-Koordinator 

Kanton Solothurn 
Bau- und Justizdepartement 
Amt für Geoinformation 
SO!GIS Koordination 
Rötistrasse 4 
CH-4501 Solothurn 

Telefon ++41(0)32 627 25 32 
Telefax ++41(0)32 627 22 14 

mailto:horst.duester at bd.so.ch 
http://www.agi.so.ch 

 

   -----Ursprüngliche Nachricht-----
   Von: nicklas.aven at jordogskog.no [mailto:nicklas.aven at jordogskog.no]
   Gesendet am: Mittwoch, 16. September 2009 10:23
   An: PostGIS Users Discussion
   Betreff: Re: [postgis-users] st_intersection error
   

   
   Hallo Horst
    
   The precision problem is not the vertexes that they have in common.
   They are exactly the same. The problem is that extra vertex that only
   appears in one of the lines. It will still not be exactly on the
   line.
    
   I think the best solution is to use a thin buffer sile
    
   select st_intersection(a.the_geom, st_buffer(b.the_geom,0.00001)),
   a.myid
   from aline1 as a, aline2 as b
    
   One problem here might be that, as st_buffer works today you will
   probably get an extra vertex in start and end of your resulting
   linestring very close to the original vertexes because the buffer
   will cause an intersection buffered in all directions (new
   possibilities in 1.5 to avoid that :-) ) 
    
   Or actually in your case you will only get that extra vertex on the
   west part because there the line 1 changes direction. On the east
   side the vertex is just moved a little bit.
    
   Hope that helps
   Nicklas
    
   2009-09-16 Düster Horst wrote:
   
   >
   Martin, Nicklas

   >> 
   Thank you for your responses. To avoid precision problems I modified
   my query to:

   >> 
   select st_intersection(st_snaptogrid(a.the_geom,0.01),
   st_snaptogrid(b.the_geom,0.01)), a.myid
   >from aline1 as a, aline2 as b

   >> 
   As the result I do get a MULTIPOINT geometry with three points. These
   points are exactly the vertex points of my expected LINESTRING
   geometry. Why not a LINESTRING?

   >> 
   Regards
   >Horst

   >> 
   ------------------------------------------------

   >> 
   Dr. Horst Düster
   >Stv. Amtschef / GIS-Koordinator 

   >> 
   Kanton Solothurn
   >Bau- und Justizdepartement
   >Amt für Geoinformation
   >SO!GIS Koordination
   >Rötistrasse 4
   >CH-4501 Solothurn

   >> 
   Telefon ++41(0)32 627 25 32
   >Telefax ++41(0)32 627 22 14

   >> 
   mailto:horst.duester at bd.so.ch
   >www.agi.so.ch

   >> 
   -----Ursprüngliche Nachricht-----
   >Von: Martin Davis [mailto:mbdavis at refractions.net]
   >Gesendet am: Dienstag, 15. September 2009 17:54
   >An: PostGIS Users Discussion; Martin Davis
   >Betreff: Re: [postgis-users] st_intersection error

   >> 
   Same result in JTS.

   >> 
   As Nicklas points out, this is a problem of precision. The software
   is 
   >working as designed, but it's not designed to use it to perform 
   >intersections with a tolerance.

   >> 
   Some more sophisticated approach is required, such as vertex snapping
   
   >the lines to a given tolerance, or perhaps using a narrow buffer and
   
   >some sort of segment-based containment tests.

   >> 
   strk wrote:
   >> On Tue, Sep 15, 2009 at 11:32:48AM +0200, Düster Horst wrote:
   >> 
   >>> strk
   >>>
   >>> I attached the geometries in my initial mail.
   >>>
   >>> Take a look at aline1.sql and aline2.sql
   >>> 
   >>
   >> I confirm a POINT and a LINESTRING in a COLLECTION
   >> is returned by ST_Intersection() with latest version:
   >>
   >> POSTGIS="1.5.0SVN" GEOS="3.2.0-CAPI-1.6.0" PROJ="Rel. 4.6.0, 21
   Dec 2007" USE_STATS
   >>
   >> Martin, how about JTS ?
   >>
   >> --strk; 
   >>
   >> Free GIS & Flash consultant/developer () ASCII Ribbon Campaign
   >> http://foo.keybit.net/~strk/services.html /\ Keep it simple! 
   >>
   >> 

   >> 
   -- 
   >Martin Davis
   >Senior Technical Architect
   >Refractions Research, Inc.
   >(250) 383-3022

   >> 
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