View source for Vector length of common boundaries

'''Q:''' Any way to get the length of common boundaries shared by neighbouring areas in a map?

'''A:'''

Sample data set: [https://geodacenter.github.io/data-and-lab/data/sids.zip sudden infant deaths data (SHP file)] from North Carolina ([https://r-spatial.github.io/spdep/articles/sids.html source1], [https://geodacenter.github.io/data-and-lab/ source2]); data imported from SHAPE file with {{cmd|v.in.ogr}}.

Let's have a look at the data:
 d.mon wx0
 d.vect sids_nc

We add a second layer to the map which references the boundaries of polygons. In the vector geometry we generate an ID (category) for each boundary:
 v.category sids_nc out=sids_nc2 layer=2 type=boundary option=add

Underlying idea: we'll fetch the IDs (categories) of the polygons left and right from each boundary and store it into the attribute table linked to layer 2. In general:
 # cat_of_boundary | cat_of_left_polygon | cat_of_right_polygon | 
 length_of_boundary

We want only one category per boundary, that's why the sides check is needed (a boundary may consist of several pieces)
So we create a new attribute table and link it to the new layer 2 of the vector map:
 {{cmd|v.db.addtable}} sids_nc2 layer=2 col="left integer,right integer,length integer"

Now we query the polygon/boundary relationships and store it into the attribute table linked to layer 2:
 {{cmd|v.to.db}} map=sids_nc2 option=sides col=left,right layer=2 

Now we have unique categories for the boundaries and can calculate the lengths:
 v.to.db map=sids_nc2 option=length col=length layer=2 

Done.

See the new attribute table containing the boundary lengths:
 {{cmd|v.db.select}} sids_nc2 layer=2

Verification (let's check boundary #193):
 {{cmd|d.vect}} sids_nc2 cat=193 layer=2 col=red type=boundary
 {{cmd|d.zoom}}
 {{cmd|d.measure}}
 # LEN:     12756.00 meters

What does the attribute table say:
 {{cmd|v.db.select}} sids_nc2 layer=2 | grep '^193'
 190|65|68|12814

This is reasonably close since on screen digitization in d.measure isn't always that precise ...

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