On Sat, Oct 28, 2017 at 3:18 PM, Huidae Cho <grass4u@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@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@gmail.com> wrote:
On Fri, Oct 27, 2017 at 2:38 PM, Huidae Cho <grass4u@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@gmail.com> wrote:
On Fri, Oct 27, 2017 at 12:52 PM, Huidae Cho <grass4u@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@gmail.com> wrote:
On Thu, Oct 26, 2017 at 5:02 PM, Markus Neteler <neteler@osgeo.org> wrote:
On Thu, Oct 26, 2017 at 9:58 AM, Stefan Blumentrath
<Stefan.Blumentrath@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@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-user
grass-user mailing list
grass-user@lists.osgeo.org
https://lists.osgeo.org/mailman/listinfo/grass-user
- all fine