On Jun 16, 2009, at 12:39 PM, Paul Kelly wrote:
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.
The inconsistent implementation is an issue certainly. I don't think
that there is a problem with underlying concept of the layer feature.
Indeed, it is a very powerful data management feature of GRASS. I just
think that another name for the feature would help users to understand
and make use of it better--especially since we also use the term
layers in the GUI layer manager to refer to superimposed displays of
distinct geospatial data files, a very common usage in GIS.
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
I agree 100%.
Michael
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