[GRASS-user] Merge spatially connected features

[Johannes Radinger]

On Thu, Mar 12, 2020 at 10:48 PM Markus Metz <markus.metz.giswork at gmail.com>
wrote:

>
>
> On Thu, Mar 12, 2020 at 12:34 PM Johannes Radinger <
> johannesradinger at gmail.com> wrote:
> >
> > Thank you Markus,
> > indeed your approach looks like what I need..The hint with
> v.net.components was the part that I was missing;
>
> Note that v.net.components does not need a network prepared with v.net,
> you can use the extract of all lines with the same stream order as it is.
>
> I just tested your suggested approach using v.net.components
(without running v.net before)...works really smoothly and very fast and I
get to the desired results of extracting subnets. Thanks a lot Markus for
this hint!

/Johannes



> Markus M
>
> > 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
> >> > >>>>
> >> > >>>> _______________________________________________
> >> > >>>> grass-user mailing list
> >> > >>>> grass-user at lists.osgeo.org <mailto:grass-user at lists.osgeo.org>
> >> > >>>> https://lists.osgeo.org/mailman/listinfo/grass-user
> >> > >>>
> >> > _______________________________________________
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> >> > https://lists.osgeo.org/mailman/listinfo/grass-user
>
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