Fwd: [GRASS-user] overlapping areas seem valid to v.build: why?

G. Allegri giohappy at gmail.com
Wed Nov 30 07:59:50 EST 2011

Mmm, sorry but I don't understand it.
The topological correctness (in a general meaning, beyond GRASS) states
that two polygons *cannot* overlap.
GRASS topological model admits overlapping areas (the build tool doesn't
complaint), but some modules produce wrong results with these areas. E.g.,
v.rast.stats collects data as if one of the areas were clipped and not
overlapping. I can explain it better, but it was just an example where
GRASS model admits a not clean area, but it silently fails...
So, GRASS is a bit "fuzzy" (*should* is an arbitrary proposition)... I
comment between your answer rows:

> In some cases, overlapping polygons can be converted to a valid grass
> vector, e.g. Landsat tiles (WRS2 coverage). The conditions are that
> polygons are only partially overlapping, each polygon has a part that
> does not overlap with any other polygon, the centroids are located in
> that non-overlapping part, boundaries are not broken at intersections,
> and it helps if nodes of boundaries are not not shared with nodes of
> boundaries of other areas.

Overlapping areas *are* valid vectors, even if they aren't topologically
correct, right?
You say "partially overlapping". What does it mean? My three polygons
overlap almost entirely (it is a synthetic layer for testing), the
centroids are located on the not-overlapping regions (i.e. the overlaps
aren't areas, nor have correctly broken boundaries) and v.build still
accept them without errors...
I don't understand where the rules you're listing are evaluated (and

> Nevertheless, even though these areas can be maintained in GRASS,
> these areas are per definition topologically not clean.

Ok, this confirms that GRASS model isn't strictly topological. It suggests
to be, to guarantee correct results (see the problem with v.rast.stats).


> Markus M
> >
> > thanks,
> > giovanni
> >
> > _______________________________________________
> > grass-user mailing list
> > grass-user at lists.osgeo.org
> > http://lists.osgeo.org/mailman/listinfo/grass-user
> >
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