[GRASS-user] Merge spatially connected features

[Johannes Radinger]

Thank you Markus,
indeed your approach looks like what I need..The hint with v.net.components
was the part that I was missing; I'll try as soon as possible and will
report back on how this works.
cheers,
Johannes

On Wed, Mar 11, 2020 at 10:16 PM Markus Metz <markus.metz.giswork at gmail.com>
wrote:

> Hi Johannes,
>
> IIUC, what you want to do is an operation that involves topological
> relations of vector geometries (connected lines) and a common attribute.
> There is no easy common recipe for this.
>
> Just a suggestion:
> for each stream order:
>   extract all lines with this stream order (v.extract)
>   identify connected lines (v.net + v.net.components)
>   update a new attribute of the original lines with the comp attribute of
> the output of v.net.components plus some offset to separate different
> stream orders
>
> HTH,
>
> Markus M
>
>
> On Tue, Mar 10, 2020 at 5:20 PM Johannes Radinger <
> johannesradinger at gmail.com> wrote:
> >
> > So...no also with GRASS-user as recipient...
> >
> > On 05.03.20 16:21, Micha Silver wrote:
> > >
> > > On 3/5/20 10:47 AM, Johannes Radinger wrote:
> > >>
> > >> Hi Micha, hi all,
> > >>
> > >> sorry for my late response...however, just today I managed to try
> > >> your approach of building polylines to connect "touching stream
> > >> lines"...but...
> > >>
> > >> On 24.02.20 16:48, Micha Silver wrote:
> > >>>
> > >>> On 24/02/2020 10:45, Johannes Radinger wrote:
> > >>>> Hi all,
> > >>>> I have a large river network dataset (lines). Now I'd to assign
> > >>>> unique categories to each group of connected lines that have an
> > >>>> attribute in common.
> > >>>>
> > >>>> For example, my rivers are categorized based on some kind of stream
> > >>>> order. I want to group all rivers that belong to stream order 2 and
> > >>>> are spatially connected; each group should get a unique category
> > >>>> value. I thought that I could first extract all rivers with a
> > >>>> particular attribute (e.g. stream order = 2) which will provide me
> > >>>> some scattered pattern of lines. Then I need a spatial join tool to
> > >>>> make subgroups of lines that are connected. How can I achieve the
> > >>>> latter? Any idea?
> > >>>
> > >
> > >
> > >>>
> > >>> Here's a procedure that might work for you. Somewhat clunky, but I
> > >>> think it gets what you want.
> > >>>
> > >>> It's based on the v.build.polylines module to connect all touching
> > >>> stream reaches. First extract each order from the stream vector into
> > >>> a new vector. Then build polylines. Patch them all together. Now you
> > >>> have a polyline vector with a single cat value for each set of
> > >>> original stream reaches that had the same order and that were
> touching.
> > >>
> > >> Unfortunately, the v.build.polylines tool does not work as it only
> > >> does not connect multiple (intersecting) lines like in a river
> > >> network. As an example I tried to build polylines from the stream
> > >> network of the NC dataset. Yous suggested approach should result that
> > >> each sub-network (i.e. river network that is not connected to another
> > >> one) should get its own ID/cat...however, v.build.polylines results
> > >> in a connected stream network that consists of multiple cats:
> > >>
> > > Maybe I misunderstood your question. The steps I tried use a
> > > stream_order column to group stream segments, then apply a new
> > > attribute "merged_id" to those stream orders that touch. i.e. that
> > > connect to the same confluence point.
> > >
> > >
> > > Here's what I get using the nc_basic_spm mapset:
> > >
> > >
> > > r.watershed elev=elevation accum=nc_facc drain=nc_fdir bas=nc_bas
> > > stream=nc_str thresh=1000
> > > r.stream.order stream_rast=nc_str direct=nc_fdir elev=elevation
> > > accum=nc_facc stream_vect=nc_streams
> > > ORDERS=`v.db.select -c nc_streams group=strahler column=strahler`
> > > echo $ORDERS
> > >
> > > # Create a new stream vector for each stream order
> > >
> > > for o in $ORDERS; do
> > >
> > >     v.extract input=nc_streams output=streams_${o}
> where="strahler=${o}"
> > >
> > >     # Give each polyline it's own cat value
> > >
> > >     v.build.polylines input=streams_${o} output=streams_${o}_polyline
> > > type=line cat=first
> > >
> > > done
> > >
> > >
> > > # patch the stream orders back together
> > >
> > > POLYLINES=`g.list vect pattern="streams*polyline" separator=comma`
> > >
> > > v.patch input=$POLYLINES output=streams_polylines
> > >
> > > v.db.addcolumn map=streams column="merged_id INTEGER"
> > >
> > >
> > > # And use v.distance to update that merged_id column from cat values
> > > in polylines vector
> > > v.distance from=streams to=streams_polylines upload=cat
> column=merged_id
> > > v.db.addcolumn map=nc_streams column="merged_id INTEGER"
> > > v.distance from=nc_streams to=streams_polylines upload=cat
> > > column=merged_id
> > >
> > > Now, all stream reaches that have the same order and are "touching"
> > > have the same merged_id. See the attached image.
> > >
> > >
> > > If that's not your purpose, then just ignore...
> > >
> > Micha thank you for your help and of course, you're fully correct!
> > Merging lines that belong to the same stream order works in this case
> > well...but this is because of the definition of the Strahler ordering
> > system, where there is only one "touching node" (i.e. river junction) of
> > two rivers of the same stream order (i.e. when two 2nd order streams
> > meet, the become a 3rd order stream). Thus your solution works because
> > of this specifics and might not work if streams are grouped based on a
> > different (ordering) system.
> >
> > I was already thinking of the next step (beyond simple Strahler): As
> > mentioned in my initial post I am dealing with "some kind" of stream
> > order. It is similar to grouped stream orders (e.g. stream order 1-2 =
> > "headwater streams"). I tried to somehow reproduce my situation based on
> > your example of the NC dataset. What I basically did was to reassign a
> > new stream order "99" to all former 1st and 2nd order streams. Then I
> > did exactly what you did in your example, and of course I don't unique
> > merged_ids for the subnetworks of touching lines (see attached Figs)
> > that all belong the the same "order" 99 (the original strahler order 3
> > works of course, see Fig.)...So is there a more general way (as said
> > something like v.dissolve but for lines/networks?):
> >
> > #####################
> > g.region raster=elevation
> >
> > r.watershed --o elev=elevation accum=nc_facc drain=nc_fdir bas=nc_bas
> > stream=nc_str thresh=1000
> > r.stream.order stream_rast=nc_str direct=nc_fdir elev=elevation
> > accum=nc_facc stream_vect=nc_streams
> >
> > #ORDERS=`v.db.select -c nc_streams group=strahler column=strahler`
> > #echo $ORDERS
> >
> > # Regroup orders 1-2 (to 99)
> > v.db.addcolumn map=nc_streams at test2 columns="strahler_groups INTEGER"
> > v.db.update map=nc_streams column=strahler_groups query_column=strahler
> > v.db.update map=nc_streams column=strahler_groups value=99
> > where="strahler=1 OR strahler=2"
> >
> > NEWORDERS=`v.db.select -c nc_streams group=strahler_groups
> > column=strahler_groups`
> > echo $NEWORDERS
> >
> > # Create a new stream vector for each stream order
> > for o in $NEWORDERS; do
> >      v.extract input=nc_streams output=streams_${o}
> > where="strahler_groups=${o}"
> >      # Give each polyline it's own cat value
> >      v.build.polylines input=streams_${o} output=streams_${o}_polyline
> > type=line cat=first
> > done
> >
> > d.vect -c map=streams_99_polyline at test2
> > #################
> >
> > Thank you very much!
> >
> > Cheers,
> >
> > Johannes
> >
> > >
> > >> v.clean --overwrite input=streams at PERMANENT output=streams_break
> > >> tool=break
> > >> v.build.polylines --overwrite input=streams_break at test
> > >> output=streams_poly cats=first type=line
> > >> d.vect -c map=streams_poly
> > >>
> > >> So what would be needed here is some kind of tool that connects all
> > >> touching lines and assigns a common category value, similar to the
> > >> v.dissolve tool for polygon features. I can imagine that such a task
> > >> might be not that uncommon also in another context? Any suggestions
> > >> how to achieve this in GRASS?
> > >>
> > >> A workaround that came into my mind was to create buffers around
> > >> lines in order to make areas out of lines. Subsequently these
> > >> touching areas can be merged using v.dissolve and the information
> > >> about the common category can be queried using v.distance.
> > >> Nevertheless, a rather cumbersome way to just assign a common
> > >> category value to all lines that are touching...
> > >>
> > >> Any further ideas?
> > >>
> > >> cheers,
> > >>
> > >> Johannes
> > >>
> > >>>
> > >>>> Cheers,
> > >>>> Johannes
> > >>>>
> > >>>> _______________________________________________
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> > >>>> https://lists.osgeo.org/mailman/listinfo/grass-user
> > >>>
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