[GRASS-user] Importing polygon maps with overlapping features

[Glynn Clements]

Maris Nartiss wrote:

> > 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".
> 
> GRASS vector model is advanced, but sometimes it fails for simple
> usage. It's one of best features is also it's point of failure.
> Vector areas support in GRASS is built around bogous assumption, that
> areas can not overlap.

That's not an assumption, it's a design decision. Vector maps are
supposed to tessellate 2D space. Any given point should always be in
exactly one area (possibly the exterior area, which is the area not
covered by any explicit areas).

> Such assumption holds true for many vector
> usages (i.e. property boundaries don't overlap), but fails for other
> vector usage patterns.
> Let's assume one GRASS user wants to create vector area map with
> suitable animal habitat areas. Does gay wolf and brown bear habitat
> areas may overlap in real world? Yes they can. Can they overlap in
> GRASS? No. User is forced to adapt semantics (habitat area) to data
> storage limitations (one map per species).

If you have multiple disjoint areas, they need to be separate vector
maps. You can no more have a point contained by multiple areas in a
vector map than you can set a single raster cell to multiple values.

-- 
Glynn Clements <glynn at gclements.plus.com>