[GRASS5] Layers in vectors

[Michael Barton]

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.

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.
>