[GRASS-dev] Re: terminology issues in grass7

[Paul Kelly]

On Tue, 16 Jun 2009, Markus GRASS 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.
>
> Only if all roads have a category value assigned in each layer,
> otherwise only roads with a category value in the given layer are
> displayed, e.g. time series or town development, layer 1 shows roads
> present in 2000, layer 2 roads present in 2001 and so on. GRASS network
> analysis makes use of it, i.e. having only a subset of features present
> in a layer, see e.g. manual for v.net.salesman, lines in layer 1, points
> in layer 2.

Thanks for the clarification - that's kind of what I had in mind too. 
Regarding what Michael was saying, with d.vect I think you need to use the 
option cats=1-999999 so only features with cats assigned are displayed.

> The idea of linking different
> pieces of information to the same set of vector objects comes more from
> the GRASS vector ability to assign the same category value to different
> objects. This concept can be combined with the layer concept, but layers
> can also be used to handle a subset of objects present in the vector
> (again, GRASS network analysis provides examples).

Good summary. And as you said, there may indeed be other applications we 
haven't thought of. It seems to me that the layer concept is so powerful 
that it's incredibly difficult to be able to "see the big picture" showing 
all its possibilities. But "layer" does seem to be a very good, 
sufficently general term to describe it.

Paul