[postgis-users] Find LineString segment index containing closest point?

Martin Davis mtnclimb at gmail.com
Wed Aug 5 10:09:14 PDT 2020

On Wed, Aug 5, 2020 at 9:38 AM Martin Davis <mtnclimb at gmail.com> wrote:

> Added to the wiki:
> https://trac.osgeo.org/postgis/wiki/UsersWikiLinearRefFunctions
> It seems a bit awkward to use this as it stands for inserting a vertex in
> a line at the closest point.  I wonder if the function should also return
> the closest point itself?  (In fact, I am certain that it should if it is
> implemented as a native function, since that additional useful information
> costs very little to obtain).

And done...  updated the wiki with a new version of the function which
returns the index, distance and computed point on line.  Here it is in
action, used for adding a vertex to a line:

WITH data(id, line) AS (VALUES
    ( 1, 'LINESTRING (0 0, 10 10, 20 20, 30 30)'::geometry )
loc AS (
  SELECT id, line, index, geom AS pt
    FROM data
    JOIN LATERAL ST_LineLocateSegment( data.line, 'POINT(15
15.1)'::geometry ) AS lls ON true
SELECT id, ST_AsText( ST_AddPoint( line, pt, index ) )
  FROM loc;

Note this is probably better done using ST_Snap, but sometimes it's useful
to do things in a more explicit way.
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