[GRASS-user] RE : Merge polygone with same attribut of a shapefile
bain at toraval.fr
Thu May 16 08:12:34 PDT 2013
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)
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
*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
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