[GRASS-dev] Re: terminology issues in grass7

[Paul Kelly]

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