[GRASS-user] Transforming raster lines to vectors with correct segment attributes

Johannes Radinger johannesradinger at gmail.com
Mon Feb 3 06:51:49 PST 2014

I already tried the v-flag. This creates a attribute table with in this
case 4 lines (corresponding to the 4 distinct FCODE values).
However there are more segments than just 4. There are e.g. multiple
"segments" with the value 33400 in the entire region
which are maybe then joint into one category? When I try then to query the
map with the query-tool nothing happens (no selection in yellow, no
information). So the desired result should contain probably more than 100
segments of which some have the same value in an attribute column called
FCODE. I don't really know how the -v flag can help here?


On Mon, Feb 3, 2014 at 1:48 PM, Moritz Lennert <mlennert at club.worldonline.be
> wrote:

> On 03/02/14 11:48, Johannes Radinger wrote:
>> Hi Moritz,
>> hi all,
>> sorry for my late response. I can now provide an example using the NC
>> dataset.
>> First I needed to generate data (rasterized line/river) similar to the
>> data input I am using.
>> So I transformed the streams vector into a raster using the attribute
>> column FCODE as
>> value for the raster (v.to.rast input=streams at PERMANENT output=r_streams
>> column=FCODE)
>> Thereafter I thinned the raster (r.thin input=r_streams at PERMANENT
>> output=r_streams_thin). The
>> output corresponds to a map similar to my original map I am using.
>> Now I wanted to (back-)transform the raster map back to lines where the
>> resulting line segments should
>> correspond to the segments visible in the raster map
>> (r.to.vect input=r_streams_thin at PERMANENT output=v_r_streams_thin
>> feature=line). However, lines
>> are broken only where different lines join.
>> I attached a screenshot that illustrates my task. You can see the raster
>> map with three
>> different segments (two different values) for that single tribuatry
>> (indicated by blue-yellow-blow).
>> However when the raster gets transformed into a line this information
>> gets lost, as lines
>> are not broken where a new raster value starts. Any suggestions how to
>> solve such
>> a task?
> Same response as last time: try the -v flag of r.to.vect.
> Moritz
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