[GRASS-user] v.extract -d vs. v.dissolve

[Hamish]

Nikos:
> I think I misunderstood the function of v.dissolve here :-(
> 
> 1. My map seems to be already "dissolved", that is: there are no
> adjacent polygons carrying the same "value"

ok, so v.dissolve is not needed

> 2. When I dissolve the already "dissolved" map based on the "values"
> column, the module reclassifies and brings the "unique" "values" in a
> cat column (right?)

Yes, essentially "v.reclass column=" is all that is run then.
I just updated the v.reclass help page to try and make it a bit clearer
about what happens then, see NOTES and EXAMPLE 2:
  https://trac.osgeo.org/grass/changeset/34237


> I guess it's ok to use my already dissolved map or just add a column and
> populate unique id's for each of the polygons.

ok, but if things were dissolved you wouldn't be able to do that and keep
it in sync with the original cat numbers.

say you have two touching polygons cat 1001 and 1002. Both are owned by
"Farmer John" and you dissolve by owner string column. The new polygon
covering both farms can not keep cat 1001 or 1002, it needs a new unique
cat value. And the attribute attached to that cat value will be all farms
owned by John.



Hamish