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

[Markus Neteler]

tlaronde at polynum.com wrote on 11/22/2006 04:10 PM:
> 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?
>
>   
The Programmer's manual should hopefully explain it:
http://mpa.itc.it/markus/grass63progman/
-> Related Pages <http://mpa.itc.it/markus/grass63progman/pages.html>
  -> GRASS 6 Vector Architecture
    -> Vector library Topology management

Markus