[GRASS-user] [GRASSLIST:1114] Re: layers in vector datasets

[David Finlayson]

I can't answer your specific question, but I think I can answer the general one.

Vector layers in GRASS lingo are equivalent to "joined" tables in
ArcGIS. In GRASS the geometry information is stored in the file system
and attributes are stored in a database. The key linking the geometry
to the table is the category (often abbreviated to cat). The category
is equivalent to the Shapefile ObjectID.

You can have multiple tables linked to a single vector file hence the
layer=number terminology.

Tools which work with the linkage between the vector and the attribute
table begin with:

v.db.*

Whereas, tools that work strictly with the database table are prefaced with:

db.*

In many cases, the db.* commands can be replaced by SQL calls sent
directly to your database via conventional interfaces/GUI (nothing to
do with grass).

On 6/17/06, M S <mseibel at gmail.com> wrote:
> (sorry if this is a bit of a cross post,  but i put a similar question under
> v.net.steiner.  since not everyone may realize its really a layer question i
> figure'd id repost and ask)
>
>  can someone simplify an explanation about layers in grass vectors?
>  i read about them on this page.
> http://www.geog.uni-hannover.de/grass/grass61/manuals/html61_user/vectorintro.html
>  its under the "attribute management" section.  it seems like they have more
> to do with linking tables?
>
>  The reason i ask, is because i'm trying to input to the v.net.steiner
> function without success.  the function (as i understand it) requires inputs
> of arc layer, and point layer, but all in same file.
>
>  when opening the "network" grass vector in QGIS, it reveals that there are
> 3 layers.  (which were built using v.patch to put them all together.)
>  layer 0 (arcs) = lines connecting buildings to streets
>  layer 1 (arcs) = streets
>  layer 1 (point) = buildings
>
>  because you specify the a(rc)layer and n(ode)layer, the fact that i have
> arcs in two layers seems like it is causing the function to bail.
>
>  is there a way to put all the arcs on one layer? or is there something else
> taht i am missing.  layers in vector data models are a foreign concept to me
> coming from the a/i workstation world.
>
>  any help or clarification would be great!  thanks in advance.
>


-- 
David Finlayson