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

Nikos Alexandris nik at nikosalexandris.net
Thu May 16 06:24:05 PDT 2013

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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