[postgis-users] st_intersection error

[Düster Horst]

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
http://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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