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

Martin Davis mtnclimb at gmail.com
Wed Aug 5 08:59:53 PDT 2020

Returning a record with all information is a nice extension.  Might choose
a different function name, though.

And agreed, it makes more sense to make the index 0-based, to match the
ST_AddPoint function.  The point index origin in PostGIS functions seems to
a be bit inconsistent - or is it only ST_PointN which is different?

On Wed, Aug 5, 2020 at 8:46 AM Bruce Rindahl <bruce.rindahl at gmail.com>

> My two cents:
> Since you are computing the distance, why not return it?  Also make the
> index zero based like most of the other st_ functions.
> CREATE OR REPLACE FUNCTION ST_LineLocateN( line geometry, pt geometry )
> RETURNS table(index integer, dist double precision)
> AS $$
>     SELECT i - 1, dist FROM (
>     SELECT i, ST_Distance(
>         ST_MakeLine( ST_PointN( line, s.i ), ST_PointN( line, s.i+1 ) ),
>         pt) AS dist
>       FROM generate_series(1, ST_NumPoints( line )-1) AS s(i)
>       ORDER BY dist
>     ) AS t LIMIT 1;
> $$
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