[GRASS-user] How to link or combine adjacent vector lines

[Markus Metz]

On Sat, Feb 11, 2012 at 7:49 PM, Stefan Luedtke <sluedtke at gfz-potsdam.de> wrote:
> Sorry for the delay... and thanks for the fast answer.
>
> Adding a column and then running a command in the sense of "v.dissolve" is
> not an option because there are just to many. I was thinking of a way that
> combines two lines into one if they share the same node. For the example
> mentioned the last time, this means, keeping the line as a single one until
> the tributary merges the main river. The main river might consist of more
> than one line, but a short tributary should not.

Maybe v.build.polylines is what you are looking for. If there are
attributes associated with the lines, the cats option should probably
be set to first or multi.

HTH,

Markus M

>
> I hope I dont miss anything obvious .... ;-)
>
> Cheers,
>
> Stefan
>
>
> On Fri, 2012-02-10 at 17:04 +0200, Micha Silver wrote:
>
> On 02/09/2012 11:59 PM, sluedtke wrote:
>
> Dear friends,
>
> I have been trying for some hours now but could not find any solution.
>
> We do have a shape file of a river network,  but a bad one because single
> rivers consist of multiple lines. Just think of the easiest example, a river
> with one tributary. I would like to end up with an vector dataset that holds
> 2 lines for this example, one for the river and one for the tributary of
> course. In my case, the dataset holds, just as an example, 10 lines, and a
> distinct set of them makes either the river or the tributary. The lines of
> each set are connected by the nodes of the single lines.
>
>
> Any ideas how to get ride of this format are very welcome.
>
>
> You might be interested in this thread from a year ago.
> http://osgeo-org.1560.n6.nabble.com/snap-point-to-line-break-line-at-given-points-tp3887413p3887413.html
>
> The short story is that GRASS builds topology when importing a line vector.
> So each intersection of two lines (i.e. a tributary that branches off the
> main channel) will be made into a separate line. You can force GRASS to
> ignore topology, but you probably don't want to.  And a correct river
> network does indeed keep each stream reach as a separate line. (Then you can
> use the various river network modules r.stream.*). So maybe your original
> shapefile is not "bad"...
>
> What you can do it add an additional column to the vector attrib table and
> indicate, in that column, which tributary each segment belongs to.  THen you
> can select, display, label, calculate, etc based on this "tributary"
> attribute column. Would that help?
>
>
> Thanks in advance,
>
> Stefan
>
>
>
> --
> View this message in context:
> http://osgeo-org.1560.n6.nabble.com/How-to-link-or-combine-adjacent-vector-lines-tp4381557p4381557.html
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> --
> Micha Silver
> GIS Consultant, Arava Development Co.
> http://www.surfaces.co.il
>
>
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