[postgis-users] Extend linestring to snap to polygon

[Shane Carey]

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

[Nicolas Ribot]

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

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[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