[GRASS-user] Problems in adding tables to a map

[Moritz Lennert]

On 17/11/12 11:42, Aldo Clerici wrote:
> GRASS user,
>
> I'm having some problem in connecting tables to a map.
>
> This is the procedure (in order to simplify I used the same table but
> with different names) :
>
> g.copy vect=roads,roads1
>
> g.copy vect=roads,roads2
>
> The two tables are in the table list.
>
> db.tables -p
>
> v.db.connect map=roads1 table=roads2 layer=2
>
> The connections seem ok:
>
> v.db.connect -p map=roads1
>
> Vector map <roads1 at esercizi> is connected by:
>
> layer <1> table <roads1> in database
> </home/clerici/GRASSdata/spearfish60/esercizi/dbf/> through driver <dbf>
> with key <cat>
>
> layer <2> table <roads2> in database
> </home/clerici/GRASSdata/spearfish60/esercizi/dbf/> through driver <dbf>
> with key <cat>
>

Contrary to common belief (and unfortunately some parameter descriptions 
in man pages still support that belief) layers are not connections to 
tables as such. You can, however, connect features that have category 
values in a given layer to a table. Please read the section on layers in 
the introduction to vector data processing in the man pages [1].

In your case, no feature has a category value in layer 2 so layer 2 is 
"empty" in terms of vector features. You first have to add category 
values to layer 2 in the vector file in order to be able to visualise 
them and to connect them to attributes in a table.

Moritz


[1] http://grass.fbk.eu/grass64/manuals/html64_user/vectorintro.html