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

Markus Metz markus.metz.giswork at googlemail.com
Wed Nov 30 03:17:44 EST 2011

G. Allegri wrote:
> Back again with my topological questions :)
> Two overlapping polygons were imported without cleaning them (-c option in
> v.in.ogr).
> The result are two overlapping areas, correctly built from two overlapping
> boundaries and their centroids.
> If I run v.build to output topological errors, the output is empty. I deduce
> that the overlapping areas are topologically correct, but the GRASS
> topological model states that two boundaries *should* not overlap.
> Question: Is the *should* proposition a suggestions or a rule to state
> boundary/area validity? From my simple test, it seems that the two areas
> overlap but yet they're correctly built (level = 2).

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.

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

Markus M

> thanks,
> giovanni
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