[GRASS-user] Importing polygon maps with overlapping features

[Vincent Bain]

Hello there,

yes this can be seen as a limitation of grass, or more precisely of the
topological data model. But I think one must remind of the strict border
that should lie between geometry and a "semantic" contents.

With a relational database structure you can of course overlap various
characteristics of a single area : on one hand you have a set of
adjacent -- that is topologically clean -- polygons, on the other hand a
table or a set of related tables holding the complexity of attributes ;
the cat value of a centroïd has a univoque link with a row of a table,
then you can define associations of polygons (ajacent, included,
non-adjacent).

Grass does not provide tools for directly manipulating let's say
'multi-polygon' objects, but the ability to link a database to simple
geometry allows the handling of complex objects.

______

(a bit out of context : for those who worked with ArcInfo, it is
sometimes frustrating not to have in Grass a feature class like
REGION-SUBCLASS, which is to my mind a far better solution than
multipolygons (the OGC standard geographic features))

Yours,
Vincent.


Le mercredi 07 janvier 2009 à 14:47 +0200, Maris Nartiss a écrit :
> Hello,
> GRASS vector model is advanced, but sometimes it fails for simple
> usage. It's one of best features is also it's point of failure.
> Vector areas support in GRASS is built around bogous assumption, that
> areas can not overlap. Such assumption holds true for many vector
> usages (i.e. property boundaries don't overlap), but fails for other
> vector usage patterns.
> Let's assume one GRASS user wants to create vector area map with
> suitable animal habitat areas. Does gay wolf and brown bear habitat
> areas may overlap in real world? Yes they can. Can they overlap in
> GRASS? No. User is forced to adapt semantics (habitat area) to data
> storage limitations (one map per species).
> 
> Probably this is not the best example, simply I could not make better one fast.
> 
> Sorry for language and trolling,
> Maris.
> 
> 2009/1/7, Benjamin Ducke <benjamin.ducke at oxfordarch.co.uk>:
> 
> >
> > The only trouble this gives me in practice arises when I need to
> > import data created in non-topological GIS (e.g. ArcView Shapefiles)
> > that contains overlapping polygons. Granted, those should not exist
> > in the first place, but bad data quality and faulty topology is a
> > constant reality in my field of work. With overlapping polygons,
> > centroids cannot always be related to exactly one polygon, so the
> > topology building fails for those cases and attribute data does
> > not get attached "correctly".
> >
> 
> > Cheers,
> >
> > Ben
> >
> >
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