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

[Nikos Alexandris]

On Tue, 2008-11-11 at 00:37 -0800, Hamish wrote:
> 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

Thanks for the extra explanations.

Regards, Nikos