[postgis-users] Postgis - Line to Point distance

Peter Kukuča pjr.kukuca at gmail.com
Fri Jul 10 04:04:59 PDT 2009 

Ok, I will be more precise.

A line in postgis is represented by a sequence of points (let's call them
A,B,C,D...) connected with straight lines, right?

Now, for me, the distance between a point (X) and a line in general is the
smallest distance between any point on the line and the point X. But, by any
point on the line, I do not mean any of the A,B,C,D... points, but also any
point on the straight lines between A and B, B and C, C and D....

So, the function you suggested - ST_line_locate_point(line, point), does it
give me one of the A,B,C,D... points or does return really the closest
point, even if it has to find it somewhere on the connecting lines? This is
crucial. If this can really find the closest point, then the rest is great,
but I doubt that.

2009/7/10 Pavel Iacovlev <iacovlev.pavel at gmail.com>

> The length of the line, you can't calculate a distance between a point
> and a line in general. You can calculate a distance between a point
> and a point on a line.
>
> ST_distance_spheroid(point, ST_line_interpolate_point(line,
> ST_line_locate_point(line, point)),
> 'SPHEROID["GRS_1980",6378137,298.257222101]')
>
> ST_line_locate_point(line, point) - a point on the line that is most
> close to your initial point
>
> ST_line_interpolate_point - get that point as a point geometry
>
> ST_distance_spheroid - calculate the length between 2 points
>
> 2009/7/10 Peter Kukuča <pjr.kukuca at gmail.com>:
> > How is your query calculating the distance between a point and a line?
> Are
> > you not just calculating the length of the line?
> >
> > 2009/7/10 Pavel Iacovlev <iacovlev.pavel at gmail.com>
> >>
> >> Works on lines for me:
> >>
> >> QUERY:
> >> select ST_length_spheroid(st_transform(the_geom, 4326),
> >> 'SPHEROID["GRS_1980",6378137,298.257222101]') as ellps_distance,
> >> st_distance(the_geom) as normal_distance from fondcart_layers.roads
> >> limit 1
> >>
> >> RESULT:
> >> ellps_distance: 9214.65310341668
> >> normal_distance: 9214.10811677981
> >>
> >> 2009/7/10 Peter Kukuča <pjr.kukuca at gmail.com>:
> >> > ST_Length_spheroid is perfectly great, but it does not take a line as
> an
> >> > input :-(
> >> >
> >> > 2009/7/10 Pavel Iacovlev <iacovlev.pavel at gmail.com>
> >> >>
> >> >> And for geometry length use ST_length_spheroid(geometry,spheroid)
> >> >>
> >> >> On Fri, Jul 10, 2009 at 11:48 AM, Pavel
> >> >> Iacovlev<iacovlev.pavel at gmail.com> wrote:
> >> >> > you can take the middle of the line,
> >> >> > st_line_interpolate_point(the_geom,
> >> >> > 0.5)
> >> >> >
> >> >> > 2009/7/10 Peter Kukuča <pjr.kukuca at gmail.com>:
> >> >> >> Hi Pedro,
> >> >> >>
> >> >> >> thank you for your reply.
> >> >> >>
> >> >> >> I cannot use pointn, the line can be up to several hundred
> >> >> >> kilometers
> >> >> >> long
> >> >> >> and consits of up to 800 points, so this would be too rough.
> >> >> >>
> >> >> >> Regardinf the decond suggestion, that is exactly what I am doing
> >> >> >> now,
> >> >> >> but I
> >> >> >> don't know, which projection (srid) to transform the wgs84 data
> >> >> >> into.
> >> >> >> It
> >> >> >> should work on all of the earth's surface, at least in all of
> europe
> >> >> >> for a
> >> >> >> start. And from what I found, the projections are usually tied to
> a
> >> >> >> small
> >> >> >> territory. I was thinking about mercator projection, but there is
> a
> >> >> >> zillion
> >> >> >> of them in postgis. And I found a post, that the one with srid
> >> >> >> 900913
> >> >> >> is
> >> >> >> working great, but I don't have that one in my postgis.
> >> >> >>
> >> >> >> 2009/7/10 Pedro Doria Meunier <pdoria at netmadeira.com>
> >> >> >>>
> >> >> >>> -----BEGIN PGP SIGNED MESSAGE-----
> >> >> >>> Hash: SHA1
> >> >> >>>
> >> >> >>> Hi Peter
> >> >> >>>
> >> >> >>> You could use the pointn(geometry, n) for the first point of the
> >> >> >>> linestring as a first approach.
> >> >> >>> As far as meters are concerned here's a little example of
> >> >> >>> transforming
> >> >> >>> the geometry to the desired projected system:
> >> >> >>>
> >> >> >>> select distance(transform(u.coordinates,srid),
> >> >> >>> transform(p.geometry,srid)) AS thedistance
> >> >> >>>
> >> >> >>> HTH,
> >> >> >>>
> >> >> >>> Pedro Doria Meunier
> >> >> >>> GSM: +351 96 17 20 188
> >> >> >>> Skype: pdoriam
> >> >> >>>
> >> >> >>>
> >> >> >>>
> >> >> >>>
> >> >> >>> Peter Kukuča wrote:
> >> >> >>> > Dear sir,
> >> >> >>> >
> >> >> >>> > I am not sure if I am writing to the correct email address. I
> >> >> >>> > have
> >> >> >>> > found a post on the postgis-users formu from *Rich Gibson *and
> >> >> >>> > this
> >> >> >>> > address was next to it.
> >> >> >>> >
> >> >> >>> > I am using postgis for a while now and it is great. It solved a
> >> >> >>> > lot
> >> >> >>> > of problems for me.
> >> >> >>> >
> >> >> >>> > However, I am now facing a problem I cannot solve. I searched
> for
> >> >> >>> > almost two days now, but I still cannot find an answer. Here is
> >> >> >>> > my
> >> >> >>> > problem:
> >> >> >>> >
> >> >> >>> > I have a database of linestrings in wgs84 projection and points
> >> >> >>> > in
> >> >> >>> > wgs84 projection. I would like to determine the distance
> between
> >> >> >>> > a
> >> >> >>> > line and a point. The ST_distance function does this very well,
> >> >> >>> > but
> >> >> >>> > it does not take the wgs84 projection into account and also,
> the
> >> >> >>> > result is in degrees. On the other hand, the
> ST_Distance_Spheroid
> >> >> >>> > does take the wgs84 into accound and does give the result in
> >> >> >>> > meters,
> >> >> >>> > but it does not accept a linestring as an input parameter.
> >> >> >>> >
> >> >> >>> > Is there any way i can the distance between a line and a point
> in
> >> >> >>> > meters from the wgs84 projected input? I do not need grat
> >> >> >>> > accuracy.
> >> >> >>> > +-5 meters is still good enough. Thank you for your tips.
> >> >> >>> >
> >> >> >>> > --
> >> >> >>> > S pozdravom (regards)
> >> >> >>> > Ing. Peter Kukuča
> >> >> >>> >
> >> >> >>> >
> >> >> >>> >
> >> >> >>> >
> ----------------------------------------------------------------------
> >> >> >>> >
> >> >> >>> > _______________________________________________
> >> >> >>> > postgis-users mailing list
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> >> >> >>>
> >> >> >>> _______________________________________________
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> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >> --
> >> >> >> S pozdravom (regards)
> >> >> >> Ing. Peter Kukuča
> >> >> >>
> >> >> >> _______________________________________________
> >> >> >> postgis-users mailing list
> >> >> >> postgis-users at postgis.refractions.net
> >> >> >> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >> >> >>
> >> >> >>
> >> >> >
> >> >> >
> >> >> >
> >> >> > --
> >> >> > http://iap.md, The future is open
> >> >> >
> >> >>
> >> >>
> >> >>
> >> >> --
> >> >> http://iap.md, The future is open
> >> >> _______________________________________________
> >> >> postgis-users mailing list
> >> >> postgis-users at postgis.refractions.net
> >> >> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >> >
> >> >
> >> >
> >> > --
> >> > S pozdravom (regards)
> >> > Ing. Peter Kukuča
> >> >
> >> > _______________________________________________
> >> > postgis-users mailing list
> >> > postgis-users at postgis.refractions.net
> >> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >> >
> >> >
> >>
> >>
> >>
> >> --
> >> http://iap.md, The future is open
> >> _______________________________________________
> >> postgis-users mailing list
> >> postgis-users at postgis.refractions.net
> >> http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> >
> >
> > --
> > S pozdravom (regards)
> > Ing. Peter Kukuča
> >
> > _______________________________________________
> > postgis-users mailing list
> > postgis-users at postgis.refractions.net
> > http://postgis.refractions.net/mailman/listinfo/postgis-users
> >
> >
>
>
>
> --
> http://iap.md, The future is open
> _______________________________________________
> postgis-users mailing list
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>



-- 
S pozdravom (regards)
Ing. Peter Kukuča
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