[GRASS5] Layers in vectors

[Radim Blazek]

On 3/3/06, Michael Barton <michael.barton at asu.edu> wrote:
> Trevor,
>
> I want to correct what seems to be a small, but possibly significant,
> misconception. I guess I didn't explain this well enough in our off-list
> discussions.
>
> There is no duplication of vector objects. What layers mean is that a SINGLE
> vector object can join with multiple tables, using different key fields (one
> key field pair for each join). A single vector object can have multiple key
> fields. Each key field permits it to establish a join with a different
> attribute table.
>
> These key fields must be unique in the attribute table, but do not need to
> be unique for the vector object. That is AFAICT, GRASS supports one-to-one
> and many-to-one joins, but not one-to-many or many-to-many joins.
>
> So ONE set of vector points, representing towns, can have a key field
> (layer1 cat) to join each point to an attribute table of that includes town
> size and population. This would produce a one-to-one join.
>
> The SAME set of vector points can have a 2nd key field (layer2 cat) that
> joins it to an attribute table of county information (county population,
> county area, etc.).  In this case, several points would have non-unique
> values in the 2nd key field (towns in the same county) making a many-to-one
> join to the county attribute table.
>
> A DIFFERENT map of vector areas, representing counties, could also join with
> the county attribute table. In this case, its key field (layer1 cat) would
> have unique values to create a one-to-one join with the county attribute
> table.
>
> Actually, any table can have more than one key field defined that allows it
> to join with different tables relationally. So this is not unusual in that
> sense.
>
> I think perhaps that the terminology is what may be confusing. These
> multiple key fields were originally calleds simply fields. But there was
> confusion with the database term fields=columns. Now they're called layers,
> but this may be confusing with the GIS concept of map layers.
>
> Perhaps we should call them "keys" or something else to make it clearer what
> these are.

The layers in GRASS correspond well to layers in OGR
if you consider GRASS vector map to be a data source.
TIGER is represented in OGR in the same way.
Also in in QGIS the GRASS layers appear on the same level
as layers from other data sources.

Radim


> Hope this clarifies things somewhat.
>
> Michael
> ______________________________
> Michael Barton, Professor of Anthropology
> School of Human Evolution and Social Change
> Arizona State University
> Tempe, AZ  85287-2402
> USA
>
> voice: 480-965-6262; fax: 480-965-7671
> www: http://www.public.asu.edu/~cmbarton
>
>
> > From: Trevor Wiens <twiens at interbaun.com>
> > Date: Thu, 2 Mar 2006 16:50:51 -0700
> > To: GRASS5 <grass5 at grass.itc.it>
> > Subject: [GRASS5] Layers in vectors
> >
> >
> > 2. Providing multiple keys against a single set of GIS objects. This
> > would be achieved by duplicating a vector layer into multiple layers
> > and using different CAT values to link to different tables.
> >
>
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