Get list of equidistant points on polygon border PostGIS

Question

I would like to get points from polygon border using PostGIS.

I've tried using ST_DumpPoints() function, like this:

CREATE OR REPLACE FUNCTION get_points_from_polygon(polygon geometry)
RETURNS SETOF geometry AS
$$
DECLARE
    point geometry;
BEGIN
    FOR point IN 
        SELECT DISTINCT 
            points.geom 
        FROM (
               SELECT (ST_DumpPoints(polygon)).* 
             ) AS points LOOP
        RETURN NEXT point;
    END LOOP;
END;
$$
LANGUAGE plpgsql ;

But it gets sometimes huge list of points (~100-500) and that is not what I need. I need to get N equidistant points on the border, like on the screenshot attached. Then, when I have these points I will analyze viewshed polygons from point as observer

I've found, how to identify is the point on the polygon border But have no idea , how to create first them and get as a list of points geometry.

What would be your suggestion to do this?

try to use: ST_ExteriorRing() and generate_series() and ST_LineInterpolatePoint()... — Cyril Mikhalchenko, May 01 '20 at 10:23
Cyril, thank you for the response! If you have some sample code it would be great, since I'am not close familiar with postgis... — akushyn, May 01 '20 at 11:03
Great answer, @CyrilMikhalchenko. That is what I was looking for. — akushyn, May 01 '20 at 18:54

score 2 · Accepted Answer · answered May 01 '20 at 21:46

This script solves your problem:

WITH tbla AS (SELECT (ST_ExteriorRing(((ST_Dump(geom)).geom))) geom FROM <table_name>), intervals AS (SELECT generate_series (0, 99) as steps) SELECT steps AS stp, ST_LineInterpolatePoint(geom, steps/(SELECT count(steps)::float-1 FROM intervals)) geom FROM tbla, intervals GROUP BY intervals.steps, geom;

Set the required number of points on the line, in my example it is 100...

score 1 · Answer 2 · answered Mar 09 '21 at 23:13

Another slightly different formulation using ST_LineInterpolatePoint. Note that the number of intervals needs to be substituted in two places.

WITH shell AS (
  SELECT ST_ExteriorRing('POLYGON ((5 5, 10 15, 17 10, 5 5))') geom
)
SELECT i, ST_LineInterpolatePoint(geom, (i-1.0)/40) pt
    FROM shell
    JOIN generate_series (1, 40) AS step(i) ON true;

Get list of equidistant points on polygon border PostGIS

2 Answers2