[GIS] ST_ClosestPoint(line, point) does not intersect line

postgisquery

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?

Best Answer

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

Related Solutions

PostGIS – How to Get the Nearest Point on a Linestring to a Given Point

ad 1) Looking at the documentation for your used functions, I'd say: "Yes, all concerned linestrings will be found."

expand(geometry, float)

This function returns a bounding box expanded in all directions from the bounding box of the input geometry, by an amount specified in the second argument. Very useful for distance() queries, to add an index filter to the query.

A && B

The "&&" operator is the "overlaps" operator. If A's bounding box overlaps B's bounding box the operator returns true.

ad 2) You should be able to achieve what you want via:

st_line_interpolate_point(linestring, st_line_locate_point(LineString, Point))

st_line_interpolate_point(linestring, location)

Interpolates a point along a line. First argument must be a LINESTRING. Second argument is a float8 between 0 and 1 representing fraction of total 2d length the point has to be located.

st_line_locate_point(LineString, Point)

Returns a float between 0 and 1 representing the location of the closest point on LineString to the given Point, as a fraction of total 2d line length. You can use the returned location to extract a Point (line_interpolate_point)

[GIS] How to find nearest line from point using PostGIS

SELECT name,ref,type,ST_Distance(ST_Buffer(r.geom,20),ST_SetSRID(ST_MakePoint(lon, lat),4326)) 
FROM roads r 
ORDER BY 4 ASC 
LIMIT 1;

For point in lon/lat coordinates:

SELECT name,ST_Distance(r.geom,ST_SetSRID(ST_MakePoint(lon, lat),4326)) FROM roads r ORDER BY 2 ASC LIMIT 1;

Best Answer

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PostGIS – How to Get the Nearest Point on a Linestring to a Given Point

[GIS] How to find nearest line from point using PostGIS

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