[GRASS-dev] Re: terminology issues in grass7

[Paul Kelly]

On Tue, 16 Jun 2009, Michael Barton wrote:

> In GRASS, displaying Layer 1 will show all objects for some vector 
> topologies, and only ID 1 and 2 for other topologies. However, by putting 
> values into cat for Layer 1, you can also display ID's 3 & 4 for Layer 1. You 
> can achieve the same effect by querying cat = 1 for Layer 2. The difference 
> is that sometimes empty cats are displayed and sometimes they are not. To me 
> this is kind of an automatic (inadvertent even) query. Some of this is only 
> semantics, but I think we all agree that semantics can be important.

IMHO these are all side-effects of the inconsistent way layers are handled 
amongst different GRASS modules. It's an implementation problem (perhaps 
caused by confusion among developers as options were added and modified 
over the years), rather than a fundamental problem with the layer concept.

If we manage to come to a full understanding of the capabilities and 
possibilities of the concept of vector layers in GRASS (which I feel this 
discussion is really helping us to work towards, for me anyway), then it 
would be an exciting project to do an audit of all vector modules and the 
way they handle layers, and tidy up all the inconsistencies so that the 
meaning of layers is much more obvious, simply from the module options and 
flags. Perhaps too radical though.

Paul