Name

ST_LineLocatePoint — Returns the fractional location of the closest point on a line to a point.

Synopsis

float8 ST_LineLocatePoint(geometry a_linestring, geometry a_point);

Description

Returns a float between 0 and 1 representing the location of the closest point on a LineString to the given Point, as a fraction of total 2d line length.

You can use the returned location to extract a Point (ST_LineInterpolatePoint) or a substring (ST_LineSubstring).

This is useful for approximating numbers of addresses

Availability: 1.1.0

Changed: 2.1.0. Up to 2.0.x this was called ST_Line_Locate_Point.

Examples

--Rough approximation of finding the street number of a point along the street
--Note the whole foo thing is just to generate dummy data that looks
--like house centroids and street
--We use ST_DWithin to exclude
--houses too far away from the street to be considered on the street
SELECT ST_AsText(house_loc) As as_text_house_loc,
	startstreet_num +
		CAST( (endstreet_num - startstreet_num)
			* ST_LineLocatePoint(street_line, house_loc) As integer) As street_num
FROM
(SELECT ST_GeomFromText('LINESTRING(1 2, 3 4)') As street_line,
	ST_Point(x*1.01,y*1.03) As house_loc, 10 As startstreet_num,
		20 As endstreet_num
FROM generate_series(1,3) x CROSS JOIN generate_series(2,4) As y)
As foo
WHERE ST_DWithin(street_line, house_loc, 0.2);

 as_text_house_loc | street_num
-------------------+------------
 POINT(1.01 2.06)  |         10
 POINT(2.02 3.09)  |         15
 POINT(3.03 4.12)  |         20

 --find closest point on a line to a point or other geometry
 SELECT ST_AsText(ST_LineInterpolatePoint(foo.the_line, ST_LineLocatePoint(foo.the_line, ST_GeomFromText('POINT(4 3)'))))
FROM (SELECT ST_GeomFromText('LINESTRING(1 2, 4 5, 6 7)') As the_line) As foo;
   st_astext
----------------
 POINT(3 4)

See Also

ST_DWithin, ST_Length2D, ST_LineInterpolatePoint, ST_LineSubstring