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

[Vincent Bain]

Sorry, I ignore how v.rast.stats works. If it preliminary turns vector
input map into a raster, there is good chance it performs it on the cat
value (so you may even not need to run a loop to discriminate individual
regions). Skimming through the manual page, it looks like stats are
performed on the basis of cats, adding columns to your table if they
don't yet exist. Other people on the list will probably give you much
better advice on this point. Perhaps you can try out things on a test
map.

Good luck,
Vincent.




 Le jeudi 16 mai 2013 à 15:19 +0000, BLANDENIER Lucien a écrit :
> Ok, but I will obtain statistics for each polygons separately and not a "global" statistic considering all the polygons of a same category? 
> 
> ________________________________________
> De : grass-user-bounces at lists.osgeo.org [grass-user-bounces at lists.osgeo.org] de la part de Vincent Bain [bain at toraval.fr]
> Date d'envoi : jeudi 16 mai 2013 17:12
> À : GRASS user list
> Objet : Re: [GRASS-user] RE : Merge polygone with same attribut of a shapefile
> 
> 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.
> 
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