[postgis-users] Postgis - Line to Point distance

[Pavel Iacovlev]

ST_line_locate_point returns the location (float) of the closest point
on a line to a specified point, read the postgis manual it's all
written there. After this you use ST_line_interpolate_point to
transform that location to a point geometry.

2009/7/10 Peter Kukuča <pjr.kukuca at gmail.com>:
> 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
>> >> >> >>> >
>> >> >> >>> >
>> >> >> >>> >
>> >> >> >>> >
>> >> >> >>> > ----------------------------------------------------------------------
>> >> >> >>> >
>> >> >> >>> > _______________________________________________
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>> >> >> >>>
>> >> >> >>> iD8DBQFKVv052FH5GXCfxAsRAqS1AJ0dfphFX3wrMuo+FdXVRTgmUzHRUACfUmvH
>> >> >> >>> 146yGHfxoVmlnt9a91rpWmE=
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>> >> >> >>> _______________________________________________
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>> >> >> >>
>> >> >> >>
>> >> >> >>
>> >> >> >> --
>> >> >> >> S pozdravom (regards)
>> >> >> >> Ing. Peter Kukuča
>> >> >> >>
>> >> >> >> _______________________________________________
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>> >> >> >>
>> >> >> >>
>> >> >> >
>> >> >> >
>> >> >> >
>> >> >> > --
>> >> >> > http://iap.md, The future is open
>> >> >> >
>> >> >>
>> >> >>
>> >> >>
>> >> >> --
>> >> >> http://iap.md, The future is open
>> >> >> _______________________________________________
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>> >> >
>> >> >
>> >> >
>> >> > --
>> >> > S pozdravom (regards)
>> >> > Ing. Peter Kukuča
>> >> >
>> >> > _______________________________________________
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>> >> > http://postgis.refractions.net/mailman/listinfo/postgis-users
>> >> >
>> >> >
>> >>
>> >>
>> >>
>> >> --
>> >> http://iap.md, The future is open
>> >> _______________________________________________
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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
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>
>
>
> --
> S pozdravom (regards)
> Ing. Peter Kukuča
>
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>



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