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

[Vincent Bain]

If you use v.in.ogr to import your source data, you can define the
column used as category :
"cnames=string[,string,...]
        List of column names to be used instead of original names, first
        is used for category column"
        
V.
        

Le jeudi 16 mai 2013 à 15:37 +0000, BLANDENIER Lucien a écrit :
> Actually v.rast.stats could do exactly what I want... the problem is that my category where not in the column "cat" but in another one... I will try to correct it !
> 
> Thanks
> 
> ________________________________________
> 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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