Create a random point geometry within the boundaries of a specified polygon. Implemented as a PostGIS function. Inputs include the_geom (name of input polygon geometry field) and maxiter (max # of iterations to try placing point within the polygon).

postgis_randomise_points.sql

-- Function: babysim_random_jobs_explode(geometry, integer)

-- DROP FUNCTION babysim_random_jobs_explode(geometry, integer);

CREATE OR REPLACE FUNCTION babysim_random_jobs_explode(the_geom geometry, maxiter integer DEFAULT 10000)
  RETURNS geometry AS
$BODY$
DECLARE
 i INTEGER := 0;
 x0 DOUBLE PRECISION;
 dx DOUBLE PRECISION; 
 y0 DOUBLE PRECISION;
 dy DOUBLE PRECISION;
 xp DOUBLE PRECISION;
 yp DOUBLE PRECISION;
 rpoint Geometry;
BEGIN
 -- find envelope
 x0 = ST_XMin(the_geom);
 dx = (ST_XMax(the_geom) - x0);
 y0 = ST_YMin(the_geom);
 dy = (ST_YMax(the_geom) - y0);
  
 WHILE i < maxiter LOOP
  i = i + 1;
  xp = x0 + dx * random();
  yp = y0 + dy * random();
  rpoint = ST_SetSRID( ST_MakePoint( xp, yp ), ST_SRID(the_geom) );
  EXIT WHEN ST_Within( rpoint, the_geom );
 END LOOP;
  
 IF i >= maxiter THEN
  RAISE EXCEPTION 'RandomPoint: number of interations exceeded ', maxiter;
 END IF;
  
 RETURN rpoint;

END;
$BODY$
  LANGUAGE plpgsql VOLATILE
  COST 100;
ALTER FUNCTION babysim_random_jobs_explode(geometry, integer)
  OWNER TO model_team;