[GRASS-user] Query: number of 'areas' reported in v.build output does not necessarily represent different geographical areas

Hamish hamish_b at yahoo.com
Wed May 8 02:22:36 PDT 2013

Richard wrote:
> With closer inspection of the displayed file, there are
> clearly 10 delineated areas, certainly not 323. The majority
> of centroids seem to sit on boundary lines.

it is probably tiny sliver areas along doubled boundaries which
need cleaning up.

typically grass maintains just one boundary between two adjoining
areas, while some other GIS maintain two full area polygons with
a doubling up where they touch. for various reasons those two
overlapping boundaries may deviate slightly during processing,
and the result is fractional overlap which shows itself when
GRASS builds the topology.

the help page for v.overlay talks about it, and has some
screenshot examples of what you're describing.
see also http://trac.osgeo.org/qgis/ticket/3126

no doubt others here will have the magic v.clean or otherwise
recipe to clean it up.


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