[GRASS-user] Merge spatially connected features

[Micha Silver]

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.

Finally, with v.distance you can upload that cat value to the original 
streams.


# Get a list of stream orders
ORDERS=`v.db.select -c streams group=strahler column=strahler`
echo $ORDERS
#1 2 3 4 5 6
# How many stream segments in original
v.info -t streams | grep lines
# lines=1420

# Now loop thru list of stream orders and extract stream segments for 
each order
for o in $ORDERS; do
         v.extract input=streams output=streams_${o} where="strahler=${o}"
         # Create polyline for each stream order
         # Line "connects" all touching stream segments
         v.build.polylines input=streams_${o} 
output=streams_${o}_polyline type=line cat=first
done

# Patch stream order polylines together
POLYLINES=`g.list vect pattern="streams*polyline" separator=comma`
echo $POLYLINES
v.patch input=$POLYLINES output=streams_polylines
v.info -t streams_polylines | grep lines
# lines=738

# Add a new column to the original streams for new ID value
v.db.addcolumn map=streams column="merged_id INTEGER"
# And use v.distance to update that column from cat values in polylines 
vector
v.distance from=streams to=streams_polylines upload=cat column=merged_id

HTH
>
> Cheers,
> Johannes
>
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-- 
Micha Silver
Ben Gurion Univ.
Sde Boker, Remote Sensing Lab
cell: +972-523-665918
https://orcid.org/0000-0002-1128-1325