[GRASS-user] grass vector model, cats and layers concept

[Markus Metz]

On Thu, Jun 6, 2013 at 12:28 PM, Nikos Alexandris
<nik at nikosalexandris.net> wrote:
> Nikos Alexandris:
>
>> > Still, I find it counter-intuitive that it's required to create a new
>> > vector map to achieve that.
>> >
>> > --%<---
>> > v.category in=wrs2_tiles_of_interest_testing at PERMANENT layer=2 option=add
>> >
>> > ERROR: Output vector wasn't entered
>> > --->%--
>
> Markus Metz wrote:
>
>> Because you need to modify vector geometries in order to add a new
>> layer. Categories and layers are first and foremost stored together
>> with the geometries. Whenever geometries are modified, the result will
>> be a new vector map (with the exception of v.edit and the digitizers).
>
> Just for (even more) completeness and understanding:  adding a new layer does
> not really change the geometries per-se (that is the topology, the shape).

No, it does change the geometry directly.

> It's the inner design of the vector data model, i.e. the fact that categories
> and layers are bound together.
>
> Is it wrong to describe it as an internal "indexing" issue or similar?

Yes, it is wrong. Categories and layers are first and foremost stored
together with the geometries. Therefore adding a new layer will modify
the geometries directly. IOW, categories and layers are part of the
(GRASS) vector geometry objects. Also, layers and categories have
nothing to do with topology. A new layer really is only added by
modifying the geometries, only after that you might want to attach a
table to that new layer.

I guess you confuse layer with database connection. The existence of a
layer is a condition for an optional database connection. A database
connection itself does not mean the layer of this connection actually
exists.

Markus M