On Mon, 15 Jun 2009, Michael Barton wrote:
GRASS vector layers are very different...and this is one of the problems with calling them layers. GRASS vector "layers" are simply cat values assigned to a single set of vector objects. A vector map of roads with layers 1 and 2 displays the same vector objects (i.e., the roads) regardless of whether layer 1 or layer 2 is chosen. The only difference between layer 1 and 2 is that the cat values assigned to each of the vector objects can differ between "layer" 1 and 2.
Please correct me if I'm wrong, but is it not possible for certain features to have cat values assigned only in certain layers, and for certain vector modules to only operate on features that have cat values assigned to them?
For example, if you had a vector map containing both roads and buildings, and in layer 1 only the roads had categories assigned to them, and in layer 2 only the buildings had categories assigned to them: then if with d.vect you selected to display only layer 2 and only features with categories, you would only see the buildings?
If I've understood correctly, then it is not quite correct to state that the same vector features will always be displayed no matter which layer is selected - many modules only operate on features with cat values assigned (or have an option to do so) and if some features only have cat values assigned in a certain layer then only those features will be "visible" to the modules operating on them when that layer is selected. IMHO the "layer" analogy fits this concept quite well.
Even less sure on this one, but I'd also suggest that it's a deficiency of the OpenGIS simple features format that each layer in a Shapefile has to have its own separate copy of the vector geometry - surely it would be more desirable to have shared geometry GRASS-style if it was technically possible?
Paul _______________________________________________ grass-dev mailing list grass-dev@lists.osgeo.org http://lists.osgeo.org/mailman/listinfo/grass-dev
