PostGIS
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`)`;
float8 +`*`+ST_LineLocatePoint*(`geography `a_linestring,
geography a_point, boolean use_spheroid = true`)`;
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 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)