[GRASS-user] [GRASSLIST:1115] Re: layers in vector datasets

[M S]

excellent.  thank you for the clarification.  the parallels to the the esri
software help too, since thats the world i'm migrating away from (after 10
years).

so you have layers that can be linked to various tables.  if one does a
v.patch command and now has two "layers" of lines, how can they both be put
on the same layer, or does that not even matter?  i guess its like if you
have a polygon coverage in workstation.  then u can build it for lines (the
.aat) and polygons (the .pat).  then u can add attributes and manipulate
them for the different attribute tables.  however, since u cant have various
"layers" of the same datatype in a coverage, i think that is where i get
confused.

in trying to run this v.net.steiner, i have lines on seperate layers.  not
sure if that is throwing the fucntion for a loop because you have to specify
the layer of arcs.

thanks for the help.

On 6/18/06, David Finlayson <david.p.finlayson at gmail.com> wrote:
>
> 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
>
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