[GRASS-user] vector segment lengths

[Markus Neteler]

On Thu, Nov 29, 2018 at 1:38 PM Moritz Lennert
<mlennert at club.worldonline.be> wrote:
> On 29/11/18 08:55, Ken Mankoff wrote:
> > Hi GRASS list,
> >
> > How do I find out the length of each segment after running v.split?
> >
> > I have this code:
> >
> > v.split -f input=v output=v_split units=meters length=100 --o
> >
> > I'd like to know the length. Presumably they are 100 m, but the last segment of each line in 'v' must be less than or equal to that.
> >
> > I tried this:
> >
> > v.db.addcolumn map=v_split column="length double precision"
> > v.to.db map=v_split type=line option=length columns=length units=meters
> >
> > When I display with 'd.vect' and click, I see both 'v' and 'v_split'. When I click on a segment I see the total length of 'v' and the 100 m (or 42.214 for the end segment) of 'v_split'. But I don't see it when I query it at the command line with:
> >
> > db.select sql='select * from v_split'
> >
> > In this case, it only reports the original few segments from 'v', not the many 100s of segments from 'v_split'. It appears both vectors are linked to the same table.
> >
> > Still (sometimes) trying to figure out the GRASS DB and layers connected to tables...
>
> v.split does not change the layer, nor the category information, nor the
> attribute table links of the original file. It just splits each line in
> segments and attributes the same category to all segments of the same
> original line. As the attribute table is linked to the features with
> their category as key, all segments originating from the same original
> line are linked to the same line in the original attribute table which
> is just copied to the new map.
>
> When you run v.to.db on this, it will add length information for each
> segment in its respective attribute line, but since all the segments of
> the same original line share the same attribute table line, it only gets
> updated once.
>
> If you want the length of each segment, you will have to attribute
> different category values to all of them. The best is to this on a
> separate layer, using v.category
>
> v.category v_split op=add layer=2 output=v_split_2
>
> then you can run your above commands on that layer:
>
> v.db.addcolumn map=v_split_2 column="length double precision layer=2
> v.to.db map=v_split_2 type=line option=length columns=length
> units=meters layer=2
>
> This info should probably somehow go into the man page of v.split.

I have made an attempt to add that (r73781):

https://grass.osgeo.org/grass77/manuals/v.split.html#notes-on-segment-length-information

Pls fix as needed, then I'll backport it.

Markus