[postgis-users] Obtaining nearest points between two linestrings

[Christian Heine]

> > I was trying to find a way to obtain the coordinates of the nearest
> > points between two linestring. I play some time with the distance,
> > line_locate_point, line_interpolate_point and other functions but I
> > didn't find a way to get the information I want. I think that what
> > I'm trying to find is a function like line_locate_point that obtain
> > the point on one linestring nearest to another linestring.

Apologies - pressed the send button too quickly - didn't read that you are
trying to compare two linestrings... Please disregard my earlier message.

Christian


> >
> > I know that there may be lot of cases in wich the "nearest point"
> > could be more than one (e.g . parallel segments, two linestring
> > with two sections of parallel segments at the minimum distance,
> > etc), but at least I want to get one (any) of this points.
> >
> > I also try a workaround creating a buffer around one of the
> > linestring using the distance between the linestrings as width,
> > finding the intersection between the buffer and the other
> > linestring and obtaining the centroid of the result, but I couldn't
> > make this works.
>
> below an old message by Micheal Fuhr in reply to a simliar problem I
> had finding the _longest_ distance between two points in a polygon.
> With some tinkering you should be able to tackle your problem.
>
> HTH,
> Christian
>
>
>
>
> Date:	Wednesday, 1 February 2006 12:57:21 PM Australia/Sydney
> From:	Michael Fuhr
> Subject:	Re: [postgis-users] Longest axis of a polygon
> To:	PostGIS Users Discussion
> Reply-To:	PostGIS Users Discussion
>
> On Tue, Jan 31, 2006 at 05:01:13PM +1100, Christian Heine wrote:
>  > > I'm not familiar with solving this problem so maybe this isn't
>  > > correct or what you need, but could you look for the pair of points
>  > > on the polygon's (simplified) exterior ring or convex hull
> separated
>  > > by the most distance?  It would be easy to iterate through the
>  > > points in a function.
>  >
>  > Thanks for the suggestion!  I looked at convexhull() and this seems
>  > to be quite ok for my purposes -
>  > How do I loop over a set of points in PostGIS/SQL?
>
> Here's a function that I've only minimally tested.  As I said, I'm
> not familiar with solving this particular problem so corrections
> or improvements are welcome.  One possible problem is that for
> certain geometries the function might return a linestring that lies
> entirely outside the geometry, except for the endpoints; a crescent
> comes to mind.
>
> CREATE FUNCTION longestaxis(geom geometry) RETURNS geometry AS $$
> DECLARE
>      hull    geometry := convexhull(geom);
>      ering   geometry;
>      len     double precision;
>      maxlen  double precision := -1;
>      maxi    integer;
>      maxj    integer;
> BEGIN
>      IF geometrytype(hull) IN ('POINT', 'LINESTRING') THEN
>          RETURN hull;
>      END IF;
>
>      ering := exteriorring(hull);
>
>      FOR i IN 1 .. npoints(ering) - 2 LOOP
>          FOR j IN i + 1 .. npoints(ering) - 1 LOOP
>              len := distance(pointn(ering, i), pointn(ering, j));
>              IF len > maxlen THEN
>                  maxlen := len;
>                  maxi := i;
>                  maxj := j;
>              END IF;
>          END LOOP;
>      END LOOP;
>
>      RETURN makeline(pointn(ering, maxi), pointn(ering, maxj));
> END;
> $$ LANGUAGE plpgsql IMMUTABLE STRICT;
>
> --
> Michael Fuhr
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>
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>


--
 Christian Heine
 Ph.D. candidate

 EarthByte Group • <www.earthbyte.org>
 School of Geosciences
 The University of Sydney
 Australia

 P +61 2 9351 4257  •  F +61 2 9036 6588
 <www.geosci.usyd.edu.au/~christian>

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