[postgis-users] ST_Split precision issue?

[Nicol Hermann]

Anybody any ideas, hints?

Thank you very much
Nicol

Am Sonntag, den 25.04.2021, 16:36 +0200 schrieb Nicol Hermann:
> Hello List,
> 
> I am trying to split a linestring at an intersection to another line
> into it's two segments.
> This works as expected with "dummy" coordinates, but fails with
> "real-
> world" coordinates.
> 
> This:
> 
> SELECT ST_AsText( ST_Split (ST_GeomFromText('LINESTRING(0 0,2 2)'),
> ST_GeomFromText('POINT(1 1)') ) ) as lsegments;
> 
> gives me the expected output:
> 
>                           lsegments                          
> -------------------------------------------------------------
>  GEOMETRYCOLLECTION(LINESTRING(0 0,1 1),LINESTRING(1 1,2 2))
> (1 Zeile)
> 
> 
> When I am trying this with real world coordinates I am always getting
> the un-splitted original linesting back.
> Sample:
> 
> SELECT ST_AsText(ST_intersection(aline, bline)) AS
> wktcut                                                               
>   
>                                                  
> FROM (SELECT
>     ST_GeomFromText('LINESTRING(8.39567520433486
> 49.4224277276901,8.39591232763397 49.4198517063953)') As aline,
>     ST_GeomFromText('LINESTRING(8.39481293779266
> 49.4206708596104,8.39717339245195 49.4208001995917)') As bline ) As
> foo;
> 
> returns
> POINT(8.39583178549713 49.4207266868819)
> 
> If I am using this in the split command
> 
> SELECT ST_AsText( ST_Split
> (ST_GeomFromText('LINESTRING(8.39567520433486
> 49.4224277276901,8.39591232763397 49.4198517063953)'),
> ST_GeomFromText('POINT(8.39583178549713 49.4207266868819)') ) ) as
> lsegments;
> 
> the result is 
> lsegments                                              
> -------------------------------------------------------------------
> ---
>  GEOMETRYCOLLECTION(LINESTRING(8.39567520433486
> 49.4224277276901,8.39591232763397 49.4198517063953))
> (1 Zeile)
> 
> which es equal to the input.
> 
> Is this a precision issue between differen postgis functions and how
> can I solve this?
> 
> If I'am adding ST_Snap it returns a linestring which only represents
> one of the 2 segments I would expect.
> 
> See:
> 
> SELECT 
>  ST_AsText( 
>   ST_Split ( 
>    ST_Snap ( 
>      ST_GeomFromText('LINESTRING(8.39567520433486
> 49.4224277276901,8.39591232763397 49.4198517063953)'),
> ST_GeomFromText('POINT(8.39583178549713 49.4207266868819)'), 1 ), 
>   ST_GeomFromText('POINT(8.39583178549713 49.4207266868819)') )
> );
> ) ) 
> as lsegments;
> 
> response is:
> 
> lsegemts                                               
> -------------------------------------------------------------------
> ---
>  GEOMETRYCOLLECTION(LINESTRING(8.39567520433486
> 49.4224277276901,8.39583178549713 49.4207266868819))
> 
> 
> Any hints, or do I have a misconception how ST_Split should work?
> 
> Thanks
> Nicol
> 
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