[GRASS-user] RE : Merge polygone with same attribut of a shapefile

[Vincent Bain]

Le jeudi 16 mai 2013 à 14:13 +0000, BLANDENIER Lucien a écrit :
> Thank you Vincent and Nikos for your answers,
> 
> Of course I mean v.dissolve and not r.dissolve...
> 
> Maybe I'm not clear enough wiht my problem... 
> 
> To explain what I want to obtain : 
> I've created such a shapfile in qgis using the tool "add part". I have then one feature with two polygone (see attached files).
> Then I've imported this shapfile into GRASS and I also see one feature with two polygones.
> 
> Is it possible to create such files with GRASS and in my case to merge different non-adjacent polygone into one polygone?

As Nikos said, grass vector topology does not allow this; no problem for
the hole in a polygon (an area inside it without centroid), but
non-contiguous areas will necessarily be considered as distinct areas.

> 
> The reason of this is I have to do statistics (v.rast.stats) with this shp and I want to consider all the features of a category for calculating the stats and the feature separated. (e.g. I want to have the mean of all pixels contained in the polygones of the same category and for each polygones separately)
> 
> regards

You could achieve your task with the help of categories. I mean, If you
assign a distinct category to each of your "multi-polygon-areas", you
can then identify/handle them individually.

e.g. run a loop on cat values, which:
*extracts polygons with cat=1 to a temporary vector, 
*uses the temp map to perform your cross-analyses between vector and
raster data,
*iterates to the next cat value, overwriting the former temp map,
*and so on till the last cat.

Grass runs this kind of sequential tasks incredibly fast on hundreds of
iterations...

Yours,
Vincent.