[postgis-users] Extend linestring to snap to polygon

[Nicolas Ribot]

(Line extension here is based on the shortest distance between line and
polygon. To extend in the direction on line segments, you can refer to this
list, message "Finding closet intersect point along direction of line",
april 10 2013)

Nicolas

On Mon, 15 Oct 2018 at 09:58, Nicolas Ribot <nicolas.ribot at gmail.com> wrote:

> Hi,
> you can extract the exterior of the polygon (st_boundary),  use linear
> referencing function st_lineLocatePoint to find the projection of line's
> start and end points then add the points to the line:
>
> with pg as (
>   select 'POLYGON((0 0, 5 0, 3 5, 0 0))'::geometry as geom
> ), line as (
>      select 'LINESTRING(1 1, 2 1, 3.5 2)'::geometry as geom
> ) select st_addPoint(st_addPoint(
>            line.geom,
>            st_lineinterpolatepoint(
>              st_boundary(pg.geom),
>              st_lineLocatePoint(st_boundary(pg.geom), st_startpoint(line.geom))),
>            0),
>           st_lineinterpolatepoint(
>              st_boundary(pg.geom),
>              st_lineLocatePoint(st_boundary(pg.geom), st_endpoint(line.geom))),
>           st_numpoints(line.geom) + 1)
> from pg, line;
>
> (a smarter way must exist, but linear referencing function are quite
> efficient to find/build points)
>
> Nicolas
>
> [image: Screen Shot 2018-10-15 at 09.56.22.png]
>
> On Mon, 15 Oct 2018 at 08:25, Shane Carey <careyshan at gmail.com> wrote:
>
>> Hi there,
>>
>> Is there a way to extend a linestring on both sides until it reaches a
>> polygon.
>>
>> Thanks.
>> --
>> Le gach dea ghui,
>> *Shane Carey*
>> *GIS and Data Solutions Consultant*
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> https://lists.osgeo.org/mailman/listinfo/postgis-users
>
>
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