Hello there, yes this can be seen as a limitation of grass, or more precisely of the topological data model. But I think one must remind of the strict border that should lie between geometry and a "semantic" contents.
With a relational database structure you can of course overlap various characteristics of a single area : on one hand you have a set of adjacent -- that is topologically clean -- polygons, on the other hand a table or a set of related tables holding the complexity of attributes ; the cat value of a centroïd has a univoque link with a row of a table, then you can define associations of polygons (ajacent, included, non-adjacent). Grass does not provide tools for directly manipulating let's say 'multi-polygon' objects, but the ability to link a database to simple geometry allows the handling of complex objects. ______ (a bit out of context : for those who worked with ArcInfo, it is sometimes frustrating not to have in Grass a feature class like REGION-SUBCLASS, which is to my mind a far better solution than multipolygons (the OGC standard geographic features)) Yours, Vincent. Le mercredi 07 janvier 2009 à 14:47 +0200, Maris Nartiss a écrit : > Hello, > 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. 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). > > Probably this is not the best example, simply I could not make better one > fast. > > Sorry for language and trolling, > Maris. > > 2009/1/7, Benjamin Ducke <benjamin.du...@oxfordarch.co.uk>: > > > > > 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". > > > > > Cheers, > > > > Ben > > > > > _______________________________________________ > grass-user mailing list > grass-user@lists.osgeo.org > http://lists.osgeo.org/mailman/listinfo/grass-user > _______________________________________________ grass-user mailing list grass-user@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-user