[GRASSLIST:7800] Re: for grass6: Are there basic data docs? (atts, cats, etc. question)

[Radim Blazek]

On 6/30/05, Bart <scarfboy at gmail.com> wrote:
> 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.

Category is sort of feature ID used to link geometry 
with attributes stored in external database table. 
 
> Am I right in thinking that each vecture feature
> inside a vectormap has zero, one or more
> layer,category tuples?

Yes.
 
> 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?

Exactly.

>  (the grass 5.1 vector
> architecture nor the 5.7 vector format and api
> doc seems to mention any layer related things)

'Layer' was originaly called 'field' but it was cunfusing.
There was no time to change the name in the library and 
API, but we changed it in module options and UI.

> 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?

Yes. 

> 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?)

Yes, <DEFANGED_layer,cat>, but layer was originaly (5.1, early 5.7)
called field. Layer and field are 2 names for identical things.
We continue to use 'layer' in >= 6.0. 

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

v.out.ogr exports geometry+attributes
(GML driver if you need ascii).

> 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)

Yes.
 
> Incidentally, how are centroids, once
> v.out.ascii'd, linked to an area?

Centroids are inside areas.

> And are they, as the grass 5.1 vector
> architecture document suggests, the
> only way to assign categories to areas?

Yes.

Radim