[postgis-users] ST_Split precision issue?

[Martin Davis]

It would probably be helpful if ST_Split was enhanced to have this same
behaviour (i.e. split at the closest point along the line).

On Fri, Apr 30, 2021 at 12:27 PM Martin Davis <mtnclimb at gmail.com> wrote:

> For working with points and lines it's probably better to use the linear
> referencing functions.  Here's a query which does what you require:
>
> WITH line(geom) AS (VALUES
> ('LINESTRING(8.39567520433486 49.4224277276901,8.39591232763397
> 49.4198517063953)'::geometry)
> )
> SELECT ST_AsText(
>   ST_LineSubstring( geom, 0, ST_LineLocatePoint(geom,
> 'POINT(8.39583178549713 49.4207266868819)'::geometry))
> ) AS part1,
> ST_AsText(
>   ST_LineSubstring( geom, ST_LineLocatePoint(geom, 'POINT(8.39583178549713
> 49.4207266868819)'::geometry), 1)
> ) AS part2
> FROM line;
>
>
> On Sun, Apr 25, 2021 at 7:49 AM Nicol Hermann <postgis at geoworld.de> wrote:
>
>> 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
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/postgis-users
>>
>
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