[postgis-users] Midpoint function?

[Rodrigo Martín LÓPEZ GREGORIO]

I don't know why I mixed my ideas. The function that I send in my previous
mail find the nearest point of a multilinestring to a given point so it has
nothing to do with the middle point in the multilinestring. Tomorrow I will
modify that function and I'll send it again. Sorry about that.

Rodrigo.

On 6/21/07, Rodrigo Martín LÓPEZ GREGORIO <
rodrigomartin at lopezgregorio.com.ar> wrote:
>
> I was testing the LineMerge() solution with different MULTILINESTRINGs and
> it will only return a LINESTRING geometry if there is no gap between
> constituent linestrings. I remember then that a few days ago I answered a
> similar question but that time the linestrings had gaps and I tryed with
> LineMerge with no luck, so that time I created a function that work with
> those MULTILINESTRINGs. I know that the LineMerge solution works fine in
> your case but I send again the code of the function I created. I know maybe
> there could be a better solution but at least it works :P
>
> [...] So I wrote a stored function (preety simple by the way) that get
> what you need. The function get two parameters... the first is a
> multilinestring, the second, your point. The function loops over all
> linestrings in the multilinestring, get the nearest one and then with that
> linestring calculates the location of the nearest point of the linestring to
> your point. To get this function working run this script and it will create
> your function (take a look at the end of the function and chanche the user
> name if necesary; mine was postgres):
>
> -- Function: multiline_locate_point(amultils geometry,apoint geometry)
>
> -- DROP FUNCTION multiline_locate_point(amultils geometry,apoint
> geometry);
>
> CREATE OR REPLACE FUNCTION multiline_locate_point(amultils geometry,apoint
> geometry)
>   RETURNS geometry AS
> $BODY$
> DECLARE
>     mindistance float8;
>     nearestlinestring geometry;
>     nearestpoint geometry;
>     i integer;
>
> BEGIN
>     mindistance := (distance(apoint,amultils)+100);
>     FOR i IN 1 .. NumGeometries(amultils) LOOP
>         if distance(apoint,GeometryN(amultils,i)) < mindistance THEN
>             mindistance:=distance(apoint,GeometryN(amultils,i));
>             nearestlinestring:=GeometryN(amultils,i);
>         END IF;
>     END LOOP;
>
> nearestpoint:=line_interpolate_point(nearestlinestring,line_locate_point(nearestlinestring,apoint));
>     RETURN nearestpoint;
> END;
> $BODY$
>   LANGUAGE 'plpgsql' IMMUTABLE STRICT;
> ALTER FUNCTION multiline_locate_point(amultils geometry,apoint geometry)
> OWNER TO postgres;
>
> Once the function is stored and available for use, you can make the query
> like:
>
> SELECT *, multiline_locate_point(the_geom,PointFromText('POINT(517651
> 2121421)', 42102))
> FROM mainroad ORDER BY Distance(the_geom,PointFromText('POINT(517651
> 2121421)', 42102)) LIMIT 1
>
> However, the query speed is a little bit slow if you do the query in that
> way. You can use the next query instead wich first gets the nearest
> multilinestring to your point and then call my function with that geometry
> instead of calling the function for all geometries:
>
> SELECT *, multiline_locate_point(the
> _geom,PointFromText('POINT(517651 2121421)', 42102)) FROM
> (SELECT *, Distance(the_geom,PointFromText('POINT(517651 2121421)',
> 42102)) as dist
> FROM mainroad ORDER BY Distance(the_geom,PointFromText('POINT(517651
> 2121421)', 42102)) LIMIT 1 ) as foo
>
> On my PC the second query takes 1/6 of the time that tooks the first one
> so the speed is much better at least here ;)
>
> Rodrigo
>
> On 6/21/07, Pradeep B V <pradeepbv at gmail.com> wrote:
> >
> > On 6/21/07, Rodrigo Martín LÓPEZ GREGORIO <rodrigomartin at lopezgregorio.com.ar
> > > wrote:
> > >
> > > As Nicolas says you should use:
> > >
> > > select line_interpolate_point(LineMerge(the_geom),0.5).
> > >
> > > you can try this and see if the point is the one you want:
> >
> >
> > It works fine.
> >
> > and here is the proof.
> >
> > select distance(startpoint(the_geom),
> > line_interpolate_point(LineMerge(the_geom),0.5)) as stdist,
> > distance(endpoint(the_geom), line_interpolate_point(LineMerge(the_geom),
> > 0.5)) as endist from testdataset limit 1;
> >
> >                     stdist         |       enddist
> >               ----------------------+----------------------
> >  0.000737003466840363 | 0.000737003466840363
> >
> > Thanks Rodrigo/Nicolas.
> >
> >  - Pradeep B V
> > www.btis.in
> >
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> >
>
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