[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.

> > 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.

 
Moritz Lennert wrote:

> I think the confusion comes from the fact that what Nikos calls
> "geometry" is the part "drawn" on the screen, i.e. the geometrical
> features visible to the user when displaying a map. Markus speaks 
about
> "geometry" in the programming sense, i.e. the programmed structure 
that
> represents the geometrical features, but which contains more than 
just
> the "visible" geometric parts (i.e. the coordinates), e.g. it also
> contains the information about the type of geometry and the possible
> layers (aka fields) and category values. See:
> 
> 
http://grass.osgeo.org/programming7/vlibFormat.html#vlibCoorFileFormat


@Moritz:  eXactly that. Milles mercis, Nikos