Vector length of common boundaries
Q: Any way to get the length of common boundaries shared by neighbouring areas in a map?
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:
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:
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
See the new attribute table containing the boundary lengths:
Verification (let's check boundary #193):
sids_nc2 cat=193 layer=2 col=red type=boundary # LEN: 12756.00 meters
What does the attribute table say:
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 ...