[GRASS-dev] v.category option=print: fails to print area's categories

[tlaronde at polynum.com]

Hello,

On Wed, Nov 22, 2006 at 12:58:08PM +0100, Martin Landa wrote:
> Hi Maciek,
> 
> 2006/11/21, Maciej Sieczka <tutey at o2.pl>:
> >Hi,
> >
> >I have notice that "v.category option=print type=area" never prints any
> >categories. Why is that so?
> >
> >Example: in spearfish there is a 'fields' vector which has 65 areas, 63
> >of them with centroids. But
> >
> >        v.category fields option=print type=area
> >
> >prints nothing; a bug?
> 
> not sure, I see vector type 'area' as a "virtual" type, so I think an
> area cannot be associated with category number. It's always centroid
> (and/or boundary) which has category number. So

FWIW, and to be able (I'm talking about myself) to have an overview of
your new vector engine here is how things were going with CERL
GRASS 4.1 vector format (and GRASS GPL <= 5.4) [this was not
explicitely stated; this is an extended description].

1) the vector element is an ARC, that is an oriented vector of vertices.

2) the vertices can be seen as control points. The nature of the one
dimension element drawned according to these control points can be
called the functional type (in KerGIS V_FTYPE_*). There was (is) only
one functional type supported: V_FTYPE_LINE (polylines, or segments).

3) the topological type of the ARC tells what _geometrical_ element to
deduce from the ARCS. Topological types are : V_TTYPE_DOT, V_TTYPE_PATH,
V_TTYPE_EDGE (were respectively DOT, LINE and AREA in legacy code).

4) the geometrical figures are deduced from the previous informations.
The geometrical figures are V_GTYPE_POINT, V_GTYPE_LINE, G_GTYPE_AREA.

5) the legacy categories are associated with _geometrical figures_ by a
_topological_ mean: a point "on" an element (nearest to this element
than to another one) [category are now in KerGIS group numbers, and
label group names à la DNS].

So, with the legacy topological vector engine, categories are precisely
associated with areas, the centroïd being the topological mean to make
the association.

How does your new vector engine define these things?

Cheers,

-- 
Thierry Laronde (Alceste) <tlaronde +AT+ polynum +dot+ com>
                 http://www.kergis.com/
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