ST_ClosestPoint(line, point) does not intersect line

Question

In my PostGIS database (PostGIS 1.5 on Postgres 8.4.1), I have two tables: roads (composed of linestrings) and crashes (composed of points). I've been trying to correlate each crash to a road, but I'm having problems getting the following to work:

SELECT ST_ClosestPoint(road.the_geom, crash.the_geom),
    ST_Intersects(ST_ClosestPoint(road.the_geom, crash.the_geom), road.the_geom)
    ST_Distance(ST_ClosestPoint(road.the_geom, crash.the_geom), crash.the_geom) AS distance
    FROM
        --Table crashes is already in SRID 4326
        (SELECT the_geom FROM crashes WHERE gid = 360) as crash,
        (SELECT ST_SetSrid(the_geom, 4326) as the_geom from roads) as road
    ORDER BY distance;

This queury should return the closest point to the crash with gid 360 on each road, but the ST_Intersects function returns false for the first result (the true closest point over all roads). Am I doing something wrong? Is there another way to link the crash to the closest road?

Answer 1

This is a problem coming from that ST_Intersects has no tolerance. Even if double precision coordinates holds a lot of decimals they form a grid where the only places for the points is in the crossings. Often the line doesn't intersect with any of those crosses and there is no way any point will intersect the line exactly. The workaround is to use st_dwithin instead with a small tolerance.

/Nicklas