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

BLANDENIER Lucien lucien.blandenier at unine.ch
Thu May 16 07:13:40 PDT 2013

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?

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)


De : grass-user-bounces at lists.osgeo.org [grass-user-bounces at lists.osgeo.org] de la part de Nikos Alexandris [nik at nikosalexandris.net]
Date d'envoi : jeudi 16 mai 2013 15:24
À : grass-user at lists.osgeo.org
Objet : Re: [GRASS-user] Merge polygone with same attribut of a shapefile

BLANDENIER Lucien wrote:

> Dear all,

Hi Lucien,

> I'm trying to merge all polygones with the same attribut of a shapefile. I
> saw it is possible to merge adjacent polygone with r.dissolve

Small correction:  it is v.dissolve (all v.* modules operate on vector maps as
opposed to r.* modules which operate on raster maps).

> but I didn't found how to do this with polygones that are not adjacent. Is
> there a way to do this?

I don't know/I don't think so.  GRASS' vector model is a topologically aware
system. This means that it "understands", exactly, adjacent geographic
components.  Being so, by design, supports a clean geometry (for example, non
duplication of features) and sound vector-based analyses.

If I get the "picture" correct, there is no possibility to topologically merge
distinct and spatially non-related geographic features.

> My goal is to minimise the number of feature into my table in order to have
> only one feature by category, but containing several polygones.

What is the reason to worry about the number of vector features in a vector
map?  The only way to minimise features is by eliminating them, which is
rather unwanted.

You might want to read

- "Vector object categories and attribute management" section in the "Vector
data processing in GRASS GIS" manual:
- "Developer's Vector FAQ" <http://grass.osgeo.org/programming7/vlibFAQ.html>

Each unique vector feature holds internally (that is, not exposed to the user)
a unique "feature id" (this is a 1:1 relationship).  The user can, however,
use the concept of Categories (another identification number), to assign a
vector feature to a category.  Categories are not unique and they _can_ be
(mis-)used to link to several vector features (this is a "1 Cat":"Many vector
features" relationship).  Note, the user can use them as being unique!  Also,
any vector feature can be linked to several category ids (this is a "1 Vector"
: "Many Categories" relationship).

The other interesting concept, in GRASS GIS, is the one of multiple layers.
The user can link multiple attribute tables by using multiple layers

So, maybe (inside GRASS GIS) you simply need to assign the same category to
many polygons?

Best, Nikos
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