[postgis-users] Cut lines with polygon

[Brian Modra]

2009/10/5 Alexander Strunck <A.Strunck at gmx.net>:
> Hello
>
> i am really new to postgis and i try to cut lines with a polygon. the sql statement is
>
> create table osm_line_in_dorfring2_5_10 as
> SELECT b.way_id, b.geom,
> Intersection ( g.geom, b.geom )
>  FROM stadtdorfbuffer2_5_10 AS g, osm_line AS b
>  WHERE IsEmpty ( Intersection ( g.geom, b.geom ) ) = FALSE;
>
> but when i look at the result the lines are not cut by the polygon. the lines go a little bit over the borders of the polygon. Why is that?

Have you checked the coords and done the maths (manually) to see if
the intersection points are wrong? Maybe its just a rendering problem
in whatever you are using to view them?

> the second problem ist when i want to do the same thing with two different polygon tables. i get the error
> NOTICE:  TopologyException: found non-noded intersection between 8.18405 48.9483, 8.18366 48.9481 and 8.18285 48.947, 8.18933 48.9551 8.18381 48.9482
> ERROR:  GEOS Intersection() threw an error!
>
> ********** Fehler **********
>
> ERROR: GEOS Intersection() threw an error!
> SQL Status:XX000

See if you can find the polygon that its throwing this for, and view
it to see if there is something obvious about the topology.

>
> i tested the polygon tables with ST_IsValid and everything seems to be all right.
>
> does someone have any suggestions?
>
> thx in advance
>
> alex
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