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

[Nikos Alexandris]

Nikos A wrote:

> >> > 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).


Nikos A:

> > 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).


Markus Metz:

> No, it does change the geometry directly.

O-K.  This is where I am confused.


> > 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.

I'll try to consult the programmer's manual or else about the vector 
data model.  I need to clarify this to myself :-).


> Therefore adding a new layer will modify the geometries directly. 
IOW,
> categories and layers are part of the (GRASS) vector geometry 
objects.

Rigth, I need to (re-)define to myself the "geometries" term.


> Also, layers and categories have nothing to do with topology.

A-ha...


> 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.

Will check the manuals...

 
> 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.

Danke Markus.

I am not really confused over the layer/db-connection stuff.  Rather, 
it is the topology - geometry - categories and layers stuff that I am 
confused about.

Nikos