[GRASSLIST:7391] for grass6: Are there basic data docs? (atts, cats, etc. question)

[Bart]

Hi again.

I'm sorry to be a bother, but I'm confused. 

I haven't started doing anything complex yet,
but want to know GRASS 6's data setup, because
I've been reading documentation and tutorials,
which have mostly succeeded in confusing me,
or not being very clear on what version they do or
do not apply to.

I'm mostly interested in what exactly can be done
with categories and attributes, and even the exact
distinction between and definition of those two.


Am I right in thinking that each vecture feature
inside a vectormap has zero, one or more
layer,category tuples?

Also, that you can (not must) create attribute
tables which are referenced by the layer, and
rows which are essentially referenced by that
<DEFANGED_layer,category> pair? (the grass 5.1 vector
architecture nor the 5.7 vector format and api
doc seems to mention any layer related things)

Is it purely up to the user to assign meaning, 
to create, modify and query with only SQL?
Is that what attributes are, and to what degree
can GRASS commands actually use these?


I found a diagram in the grass6 documentation
called something grass51, and an explanation
with grass5.7, so am I right in thinking all
the above (including layers) was introduced and
hasn't changed much since 5.1? 5.7?


I'm interested in exporting data with 
v.out.ascii. I assume the category lines
are <DEFANGED_layer,cat> pairs. (or are they field,cat?)

I suppose any attribute exporting actually
needs human intervention because they
defined the actual meaning.


Also, about the features themselves:
I assume that v.out.ascii outputs
level 1 data (ie. no islands and partial
arcs, but separate areas and such)


Incidentally, how are centroids, once 
v.out.ascii'd, linked to an area? 
And are they, as the grass 5.1 vector
architecture document suggests, the
only way to assign categories to areas?



Thanks in advance,
--Bart Alewijnse