[GRASS-user] convert vector line topology to area topology

[Maciek Sieczka]

On Fri, 07 Jul 2006 14:37:06 +0200
Nicolas Devaux <devaux at ensam.inra.fr> wrote:

> Hi, I start to use Grass, and I'm wondering if I'm not doing wrong, 
> trying to apply ArcInfo 7 concepts to Grass vector topology.
> 
> Is it possible to change a vector line topology map to a new vector
> area topology map?
> 
> I explain : I imported a raster map showing fields boundaries. I want
> to convert this map into vector information showing fields as areas.
> Before doing that, I would like to work on line geometry in order to
> enhance final quality information (close interrupted boundaries,
> erase dangle nodes...).
> I use r.to.vect with line option and do all cleaning operations
> needed. But after all I can't reach area topology (using v.build or
> v.clean). If I use at the beginning r.to.vect with area option,
> unclosed fields boundaries are not converted....
> With Arcinfo, I used to import at first line information, and it was 
> possible after to reach polygon topology using "build polygon"
> command. How can I manage the same problem with Grass?

Grass "areas"="boundary"+"centroid". Atrributes are attached to
centroids, boundaries are usually attribute- and even category-less.

So the way to go might be:

Once you have your vector lines file cleaned as needed, remove it's
datatable (v.db.droptable), delete categories from lines (v.category
option=del) convert them into boundaries (v.type), add new datatable,
which you will use for attributting centroids (v.db.addtable/db.execute
+v.db.connect; the latter will let you understand Grass & DB issues
better), digitize centroids, one per each area (v.digit/QGIS) adding
attributes manually (but I'm wondering whether v.what.rast wouldn't let
you sample raster at centroids location instead of manally inputting
attributes; or does it really support only points, as the manuall says
- Anybody knows?).

Maciek

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