[GRASS-user] Importing polygon maps with overlapping features

[Benjamin Ducke]

Thanks for your clarification, Moritz, that makes perfect sense
from the data model point of view.

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".

Are there any bright ideas on how to allow v.in.ogr to import maps
with overlapping polygons and still manage to create a GRASS map
that has all area objects and the right attribute table row linked
to each one of them? Export would also need to work in the same way.
My gut feeling is some patching of v.[in|out].ogr would be required...

Cheers,

Ben


Moritz Lennert wrote:
>>>
>>> To be quite honest, I have always been a bit bewildered about the
>>> choice of using a centroid point for linking attributes to area
>>> features. Could anyone here fill me in on what advantage that has?
> 
> In a topological model where a boundary is boundary of two adjacent
> polygons, you cannot link polygon attributes to the boundary as there
> would be ambiguity as to which polygon these attributes are referring
> to. So you need some way of unambiguously identifying the polygon. A
> pseudo-centroid (i.e. not the geometric centroid, but one that in all
> cases lies within the polygon) is one way of doing it and the one chosen
> in GRASS's vector model. There might be other ways, but I'm no expert on
> that.
> 

-- 
Benjamin Ducke
Senior Applications Support and Development Officer

Oxford Archaeological Unit Limited
Janus House
Osney Mead
OX2 0ES
Oxford, U.K.

Tel.: ++44 (0)1865 263 800
benjamin.ducke at oxfordarch.co.uk




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