[GRASS-user] Importing polygon maps with overlapping features

[Moritz Lennert]

On Wed, January 7, 2009 12:58, Benjamin Ducke wrote:
> 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...

The question is what you want to do with the data. If it is for mapping,
you can just use v.external. But if you want to actual do GIS analysis,
the ambiguities created by the overlaps have to be lifted at one point.
Overlapping polygons is a fundamental error in the classical 2D model as
for a particular point in space, you cannot say for sure which attributes
are linked to it.

v.clean tries to repare this (v.in.ogr uses the same routines and you can
parameter them), but in the end it is the responsibility of the user to
make the decisions...

So, I'm not sure it can be easily handled by a "patch" to v.in.ogr.

Moritz