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...
v.clean --overwrite input=streams@PERMANENT output=streams_break
tool=break
v.build.polylines --overwrite input=streams_break@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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--
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
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