[GRASS-user] merging 2 line segments in a vector

[Michael Fecht]

Hello Maciek,

Zitat von Maciek Sieczka <werchowyna at epf.pl>:

> On Thu,  8 Jun 2006 19:36:04 +0200
> Michael Fecht <Michael.Fecht at uibk.ac.at> wrote:
>
> > Hello Stephan,
> > > > > > i digitized a line layer in grass6.1cvs(from last week). Now I
> > > > > > want to merge 3 linesegments in the layer to be one segment.
> > > > > > Is there a methode/tool to do this topological task (I could
> > > > > > not find any in v.clean)?
> > > > >
> > > > >
> > > > > try v.build.polylines
> > > > >
> > > > Thanks, the geometry/topology was modified as expected. But there
> > > > is a problem with the attributes.
> > > >
> > > > The input map has a table with attributes but v.build.polylines
> > > > does not write a table for the output map as described in the
> > > > manual. Any ideas?
> > >
> > > AFAIK the table does not get altered, because the program does not
> > > know which attribute will be assigned to the new vector.
> > It should be possible!
> >
> > Extracted from the manual:
> > If the lines that make up a polyline have different attribute values
> > then v.build.polylines will set the attribute value of the polyline
> > to that of the last line  (this is the behaviour of v.build, which is
> > used to assign the attribute values).
>
> I'm not sure if the manual reflects the current v.build.polylines
> state. It looks like a copy/paste from the old Grass 5 manual.
>
> I suppose this paragraph might reffer to category numbers (a vector
> feature), not to attributes stored in table.
>
> Try hhis:
>
> Copy the original table (db.copy) and connect the copied one with
> v.build.polylines output (v.db.connect).
>
Sounds good but did not work. Probably because all features in my
v.build.polylines output have the category number 0. But there is no cat 0 in
the original table. So a connection with v.db.connect "fails".

Cheers
Michael

> Now you can also get rid of redundant datatable rows by v.extract with
> only input and output specified (which will output your vector copy with
> a table containing only rows which have a corresponding vector feature).
>
> Maciek
>
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>