[postgis-users] Make a shorter line from a given line

[pcreso at pcreso.com]

It sounds like you could use the linear referencing capability, along the lines of (untested, just off the top of my head)...

select 
makeline(st_line_interpolate_point(geom,1-((st_length(geom)-100)/st_length(geom))),endpoint(geom))


ie, make you new line the percentage along the original line required by your offset for the start point, and the endpoint remains the same.


HTH,

  Brent Wood


--- On Thu, 9/10/09, Daniel Grum <daniel.grum at unibw.de> wrote:

> From: Daniel Grum <daniel.grum at unibw.de>
> Subject: Re: [postgis-users] Make a shorter line from a given line
> To: nicklas.aven at jordogskog.no
> Cc: "PostGIS Users Discussion" <postgis-users at postgis.refractions.net>
> Date: Thursday, September 10, 2009, 5:50 AM
> nicklas.aven at jordogskog.no
> schrieb:
> > look at st_scale
> > http://www.postgis.org/documentation/manual-1.4.1SVN/ST_Scale.html
> >  or maybe st_transscale
> > http://www.postgis.org/documentation/manual-1.4.1SVN/ST_TransScale.html
> > /Nicklas
> >   
> > 2009-09-08 Daniel Grum wrote:
> > 
> > Hy,
> > >
> > >I want to make a new line from a given line
> -->both geometry types are
> > >MULTININESTRING
> > >
> > >Every step the new line should be "x meters"
> shorter than the original.
> > >
> > >I get the length of the line, f.e. 5000 meters
> (length(the_geom)) and
> > >after the step the new Line should be only 4000
> meters.
> > >
> > >I have a idea but it isn't ready:
> > >
> > >UPDATE table
> > >SET the_geom = (SELECT
> MakeLine(StartPoint(line.the_geom),
> > >EndPoint(line_merge(line.the_geom)))
> > > FROM public.table line
> > > ORDER BY line.gid);
> > >
> > >But in if I do so I have no chance to say that the
> new line has to be
> > >100meters shorter than the original line.
> > >
> > >Are there any ideas how I can solve this problem?
> > >
> > >Thanks,
> > >--Daniel
> > >_______________________________________________
> > >postgis-users mailing list
> > >postgis-users at postgis.refractions.net
> > >http://postgis.refractions.net/mailman/listinfo/postgis-users
> > >
> > >
> That is not what I'm searching for.
> 
> Becaus the line lies somewhere in the 2D space and the end
> point of the line anytime will be the same in my case...only
> the starting point should be changed.
> And it should be changed automaticly, step by step, and the
> new starting point hvave to be on the old
> line/route-->like a route in a map and every time step
> the route will be shorter, because the unit has solved x
> meters.
> 
> Do you know what I mean?
> 
> Scale and TransScale isn't the right finction I think.
> --daniel
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