[GRASS-user] Merging connected stream lines

Thu Nov 2 04:22:53 PDT 2017

I studied strongly vs. weakly connected components little bit. So...

1. As long as there are positive costs in both arc_column and
arc_backward_column between nodes, they are in the same strongly connected
components.
2. If any of arc_column and arc_backward_column is positive between nodes,
they are in the same weakly connected components.
3. If both columns are negative between two nodes, they are disconnected
even if there is a line between them.
4. Because of point 3 above, using and not using both columns may result in
different weakly connected components. No connected lines vs. no traversal
using a negative cost with connected lines.

Hope my understanding is correct.

Thank you.
Huidae

On Sun, Oct 29, 2017 at 12:07 PM, Markus Metz <markus.metz.giswork at gmail.com
> wrote:

>
>
> On Sat, Oct 28, 2017 at 3:18 PM, Huidae Cho <grass4u at gmail.com> wrote:
> >
> > BTW, let me understand weakly vs strongly connected components generated
> by v.net.components.
> >
> > Strongly connected: All paths between nodes u and v should have the same
> direction.
>
> From the manual:
> "there are directed paths from u to v and from v to u"
>
> You can create directed graphs by assigning costs according to line
> direction with the arc_column and arc_backward_column options. The forward
> or backward direction of a line can be closed by setting costs to -1.
>
> By default, (no cost columns given), line lengths are used as costs,
> ignoring line direction because 1) line direction can be arbitrary, 2)
> typically lines can be traversed in both directions.
>
> Therefore, method=strong only makes sense if both arc_column and
> arc_backward_column are given and if some lines are closed in one
> direction. Otherwise, strong and weak will give the same results.
> >
> > Weakly connected: Only requires "connections" between nodes u and v, and
> their directions don't matter.
>
> To be precise, u must be reachable from v or v must be reachable from u.
> >
> > If my understanding is correct, a stream vector map generated by
> thinning and converting the stream raster map (flow directions not
> guaranteed) to vector would only be weakly connected.
> >
> > The stream vector output from r.streams.extract would be both strongly
> and weakly connected because it honors flow directions.
>
> Different stream networks are always different components. If you use
> arc_backward_column and close backward traversal, each stream network is
> again broken up into various strongly connected components.
>
> If you use r.thin + r.to.vect + arc_backward_column, a single stream
> network might even be broken up into several weakly connected components.
>
> HTH,
>
> Markus M
> >
> > Please let me know if I'm mistaken.
> >
> > Thanks.
> > Huidae
> >
> >
> >
> > On Sat, Oct 28, 2017 at 9:06 AM, Huidae Cho <grass4u at gmail.com> wrote:
> >>
> >> It's working great! Thanks for the quick implementation. It will help a
> lot!
> >>
> >> Huidae
> >>
> >> On Fri, Oct 27, 2017 at 3:58 PM, Markus Metz <
> markus.metz.giswork at gmail.com> wrote:
> >>>
> >>>
> >>>
> >>> On Fri, Oct 27, 2017 at 2:38 PM, Huidae Cho <grass4u at gmail.com> wrote:
> >>> >
> >>> > [...] I tried your r.stream.extract to extract stream lines and its
> output makes much more sense. It would be great if this module also outputs
> unique stream "network" numbers.
> >>>
> >>> Unique stream network numbers are added as new attribute "network" in
> trunk r71599.
> >>>
> >>> Markus M
> >>> >
> >>> > Thanks.
> >>> > Huidae
> >>> >
> >>> >
> >>> > On Fri, Oct 27, 2017 at 8:14 AM, Markus Metz <
> markus.metz.giswork at gmail.com> wrote:
> >>> >>
> >>> >>
> >>> >>
> >>> >> On Fri, Oct 27, 2017 at 12:52 PM, Huidae Cho <grass4u at gmail.com>
> wrote:
> >>> >> >
> >>> >> > Actually, I'm getting the same error as Markus in the NC dataset.
> I recently discleaned and rebuilt the latest SVN.
> >>> >>
> >>> >> In trunk, this bug appeared with r71267 and was fixed with r71477.
> >>> >> In relbr72, this bug appeared with r71268 and was fixed with r71478.
> >>> >>
> >>> >> Please check your revision.
> >>> >>
> >>> >> Markus M
> >>> >>
> >>> >> >
> >>> >> > Huidae
> >>> >> >
> >>> >> > On Thu, Oct 26, 2017 at 4:14 PM, Markus Metz <
> markus.metz.giswork at gmail.com> wrote:
> >>> >> >>
> >>> >> >>
> >>> >> >>
> >>> >> >> On Thu, Oct 26, 2017 at 5:02 PM, Markus Neteler <
> neteler at osgeo.org> wrote:
> >>> >> >> >
> >>> >> >> > On Thu, Oct 26, 2017 at 9:58 AM, Stefan Blumentrath
> >>> >> >> > <Stefan.Blumentrath at nina.no> wrote:
> >>> >> >> > > Hi,
> >>> >> >> > >
> >>> >> >> > > Did you try:
> >>> >> >> > >
> >>> >> >> > > https://grass.osgeo.org/grass72/manuals/v.net.
> components.html
> >>> >> >> > >
> >>> >> >> > > That might give you a common attribute for all connected
> lines in your
> >>> >> >> > > network…
> >>> >> >> >
> >>> >> >> > I just tried that with NC but:
> >>> >> >> >
> >>> >> >> > GRASS 7.3.svn (nc_spm_08_grass7):~ > v.net.components
> input=roadsmajor
> >>> >> >> > output=roadsmajor_components method=strong
> >>> >> >> > Building graph...
> >>> >> >> > Registering arcs...
> >>> >> >> >  100%
> >>> >> >> > Flattening the graph...
> >>> >> >> > ERROR: GngFlatten error
> >>> >> >> >
> >>> >> >> >
> >>> >> >> > GRASS 7.3.svn (nc_spm_08_grass7):~ > v.net.components
> input=railroads
> >>> >> >> > output=railroads_components method=strong
> >>> >> >> > Building graph...
> >>> >> >> > Registering arcs...
> >>> >> >> >  100%
> >>> >> >> > Flattening the graph...
> >>> >> >> > ERROR: GngFlatten error
> >>> >> >> >
> >>> >> >> > Does the network need any special treatment?
> >>> >> >>
> >>> >> >> I can not reproduce these errors. Please update your local copy:
> >>> >> >> make distclean
> >>> >> >> svn --no-ignore status
> >>> >> >> svn up
> >>> >> >> configure
> >>> >> >> make
> >>> >> >>
> >>> >> >> Markus M
> >>> >> >> >
> >>> >> >> > Markus
> >>> >> >> > _______________________________________________
> >>> >> >> > grass-user mailing list
> >>> >> >> > grass-user at lists.osgeo.org
> >>> >> >> > https://lists.osgeo.org/mailman/listinfo/grass-user
> >>> >> >>
> >>> >> >>
> >>> >> >> _______________________________________________
> >>> >> >> grass-user mailing list
> >>> >> >> grass-user at lists.osgeo.org
> >>> >> >> https://lists.osgeo.org/mailman/listinfo/grass-user
> >>> >> >
> >>> >> >
> >>> >
> >>> >
> >>
> >>
> >
>
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