wiki:UsersWikiNearest

Nearest Neighbor Search

Describe Nearest Neighbor Search here. These are helper functions for finding nearest neighbors to a given geometry.

Examples of how to use these and more detailed description can be found here http://www.bostongis.com/PrinterFriendly.aspx?content_name=postgis_nearest_neighbor_generic

CREATE OR REPLACE FUNCTION expandoverlap_metric(a geometry, b geometry, maxe double precision, maxslice double precision)
  RETURNS integer AS
$BODY$
BEGIN
    FOR i IN 0..maxslice LOOP
        IF expand(a,maxe*i/maxslice) && b THEN
            RETURN i;
        END IF;
    END LOOP;
    RETURN 99999999;
END;
$BODY$
LANGUAGE 'plpgsql' IMMUTABLE;

CREATE TYPE pgis_nn AS
   (nn_gid integer, nn_dist numeric(16,5));

CREATE OR REPLACE FUNCTION _pgis_fn_nn(geom1 geometry, distguess double precision, numnn integer, maxslices integer, lookupset varchar(150), swhere varchar(5000), sgid2field varchar(100), sgeom2field varchar(100))
  RETURNS SETOF pgis_nn AS
$BODY$
DECLARE
    strsql text;
    rec pgis_nn;
    ncollected integer;
    it integer;
--NOTE: it: the iteration we are currently at
--start at the bounding box of the object (expand 0) and move up until it has collected more objects than we need or it = maxslices whichever event happens first
BEGIN
    ncollected := 0; it := 0;
    WHILE ncollected < numnn AND it <= maxslices LOOP
        strsql := 'SELECT currentit.' || sgid2field || ', distance(ref.geom, currentit.' || sgeom2field || ') as dist FROM ' || lookupset || '  as currentit, (SELECT geometry(''' || CAST(geom1 As text) || ''') As geom) As ref WHERE ' || swhere || ' AND distance(ref.geom, currentit.' || sgeom2field || ') <= ' || CAST(distguess As varchar(200)) || ' AND expand(ref.geom, ' || CAST(distguess*it/maxslices As varchar(100)) ||  ') && currentit.' || sgeom2field || ' AND expandoverlap_metric(ref.geom, currentit.' || sgeom2field || ', ' || CAST(distguess As varchar(200)) || ', ' || CAST(maxslices As varchar(200)) || ') = ' || CAST(it As varchar(100)) || ' ORDER BY distance(ref.geom, currentit.' || sgeom2field || ') LIMIT ' ||
        CAST((numnn - ncollected) As varchar(200));
        --RAISE NOTICE 'sql: %', strsql;
        FOR rec in EXECUTE (strsql) LOOP
            IF ncollected < numnn THEN
                ncollected := ncollected + 1;
                RETURN NEXT rec;
            ELSE
                EXIT;
            END IF;
        END LOOP;
        it := it + 1;
    END LOOP;
END
$BODY$
LANGUAGE 'plpgsql' STABLE;

CREATE OR REPLACE FUNCTION pgis_fn_nn(geom1 geometry, distguess double precision, numnn integer, maxslices integer, lookupset varchar(150), swhere varchar(5000), sgid2field varchar(100), sgeom2field varchar(100))
  RETURNS SETOF pgis_nn AS
$BODY$
    SELECT * FROM _pgis_fn_nn($1,$2, $3, $4, $5, $6, $7, $8);
$BODY$
  LANGUAGE 'sql' STABLE;

Notes

Though fairly fast, this method is somewhat approximate, since it uses a bounding box to determine near features, so not all features in the box can be the same distance away. A more accurate, but slower method would be to ament the expandoverlap_metric function to use ST_DWithin() instead of expand(), so replacing

IF expand(a,maxe*i/maxslice) && b THEN

with

IF ST_DWithin(a,b,maxe*i/maxslice) THEN

Last modified 8 years ago Last modified on Apr 14, 2009 1:55:45 PM