[postgis-users] line_interpolate_point does not return a point that intersects the line

[Francis Markham]

Mmm, so it is a floating point issue - the distance is 9.57711783686133e-12
meters.  Is there anyway to specify a tolerance for ST_Intersects and
similar predicates?

-Francis

On 3 May 2010 17:53, strk <strk at keybit.net> wrote:

> On Mon, May 03, 2010 at 04:41:09PM +1000, Francis Markham wrote:
> > Hi all,
> >
> > I think I've got a bit of a problem with snapping points to lines.  I had
> > thought that using line_interpolate_point(..., line_locate_point(...))
> would
> > do the trick, but this seems not to be the case:
>
> Francis,
> thing is that the constructive functions have to deal with representability
> of numbers which means you won't always get the exact point of intersection
> but rather the best approximation of it.
> Instead the predicates (like ST_Intersects) give you the exact answer.
> You may try the distance functions to see how much the intersection point
> is off the line, should be a really low number.
>
> --strk;
>
> > gis=# SELECT ST_AsText(v.the_geom), ST_SRID(v.the_geom),
> > ST_AsText(r.the_geom), ST_SRID(r.the_geom) FROM road_segments AS r,
> venues
> > AS v WHERE v.venue_id = 29 AND r.gid = 100982;
> >
> >                 st_astext                 | st_srid
> > |
> > st_astext                                    | st_srid
> >
> ------------------------------------------+---------+---------------------------------------------------------------------------------+---------
> >  POINT(415185.606066865 7827721.52951473) |   28353 |
> > LINESTRING(415250.679102704 7827994.85292521,415218.694181662
> > 7827642.44920486) |   28353
> > (1 row)
> >
> > gis=# SELECT ST_Intersects(ST_line_interpolate_point(r.the_geom,
> > ST_line_locate_point(r.the_geom, v.the_geom)), r.the_geom) FROM
> > road_segments AS r, venues AS v WHERE v.venue_id = 29 AND r.gid = 100982;
> >
> >  st_intersects
> > ---------------
> >  f
> > (1 row)
> >
> > gis=# SELECT postgis_full_version();
> >
> >
> > postgis_full_version
> >
> ----------------------------------------------------------------------------------------
> >  POSTGIS="1.4.1" GEOS="3.2.1-CAPI-1.6.1" PROJ="Rel. 4.7.1, 23 September
> > 2009" USE_STATS
> > (1 row)
> >
> >
> >
> > Any assistance would be much appreciated.
> >
> > Thanks,
> >
> > -Francis Markham
>
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>
>
> --
>
>  ()   Free GIS & Flash consultant/developer
>  /\   http://strk.keybit.net/services.html
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