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)

Hallo

First the question about what ST_Distance returns. ST_Distance returns the shortest distance between the line and the point (or what geometry types are inputed) That means that ST_Distance between point (1 3) and linestring (0 0,0 10) will return 1. The distance will not be measured between the point and (0 0) or the point and (0 10) but from the point (1 3) to (0 3).

So from what I understand ST_Distance gives you the answer you want.

If you want to find the point (0 3) in the example above you can use ST_Closestpoint if you have PostGIS 1.5 For my example you use it like this: ST_Closestpoint('LINESTRING(0 0,0 10)'::geometry, 'POINT(1 3)'::geometry) then you should get the point (0 3) in return, the point on the line that is closest to your point.

HTH Nicklas

I found it :) (Well i guess :P)

Using the ST_Line_Locate_Point() and ST_Line_Interpolate_point() I managed to get a point which IS NOT part of the LINESTRING definition but IS on the said line :) All I have to do is to get the distance from my point to this point and I'm done.

SELECT AsText(ST_Line_Interpolate_Point(myLineGeom,ST_Line_Locate_Point(myLineGeom,ST_Transform(GeomFromText('POINT(LON LAT)',4326),3395))))
FROM myLines
WHERE myGeom && expand(ST_Transform(GeomFromText('POINT(LON LAT)',4326),3395), 100)

The ST_Line_Locate_Point() method find the location of the closest point on the line to the given point, the ST_Line_Interpolate_Point method turn this location into a point.