[GRASS5] Layers Clarification

[Radim Blazek]

You cannot link one layer to 2 tables but you can link
one geometry object to 2 tables. I.e. the geometry must
have 2 categories in different layers.
That means you can link objects in one map to more tables
but objects in one layer are always linked to one table only.

Radim

On 3/6/06, Moritz Lennert <mlennert at club.worldonline.be> wrote:
> Trevor Wiens wrote:
> > Thanks you all for your comments.
> >
> > I want to make sure I understand so I will reiterate what I
> > understand you to have said.
> >
> > 1. What is a vector file "layer".
> >
> > In a GRASS vector file there is a single collection of spatial objects.
> > Those spatial objects can be associated with one or more keys (called
> > layers in current terminology). The use of these multiple keys allow
> > for grouping of topologically related objects in a single layer that
> > are thematically different (forests and lakes or lines and nodes). This
> > also allows for linking different series of attribute data to a single
> > object.
>
> Taking the risk of confusing things a bit more, but I do want to try to
> understand. In an exchange with Radim on layers in September, we came to
> the following conclusion of the discussion:
>
> ***********
> On 9/16/05, Moritz Lennert <mlennert at club.worldonline.be> wrote:
>  > Ok, so it was a problem of me not understanding layers correctly.
>  > Sorry...
>  > Do I then understand correctly that you cannot link one layer to
>  > different tables simultaneously ?
>
> Yes, you cannot.
>
> Radim
> **************
>
> This seems to contradict what Trevor write above, i.e. "This also allows
> for linking different series of attribute data to a single object."
>
> So if I want to link different data tables simultaneously to the same
> object, I have to duplicate the object in two layers, or ?
>
> The idea of layers, as I understood it so far, is to be able to have
> different objects linked to separate tables, but all in the same
> topological file.
>
> Moritz
>