[GRASS-user] RE: Problem querying layers other than '1' in gi s.m

[Radim Blazek]

On 9/26/06, Moritz Lennert <mlennert at club.worldonline.be> wrote:
> Radim Blazek wrote:
> > On 9/22/06, Moritz Lennert <mlennert at club.worldonline.be> wrote:
> >> > There are more, examples:
> >> > - Overlapping areas, e.g. a road on a bridge and a road below
> >> > - Many records with different time for the same point
> >>
> >> Both of these examples use different layers to store information, not
> >> different category values in the same layer, or ?
> >
> > No. One layer, more cats per geometry.
>
> What's the advantage of doing this in one layer ? Why not use one layer
> with one category value and store different time records for the same
> category in the one table linked to that layer ?

That would be also possible in theory but GRASS does not work in this manner.
The reason is that this way it is not possible to model for example overlapping
areas. Imagine  2 areas, bridge over another road, each road has one
record in table.

Map:
           +-------+
           |   A   |
+---------+--------+-----------+
|   B     |  A/B  |   B       |
+---------+--------+-----------+
           |   A    |
           +--------+

Table:
+------+--------+
|  cat |  road |
+------+--------+
|   1   |   A   |
+------+--------+
|   2   |   B   |
+------+--------+

Now try to fill in cats to the map. It is impossible. You need to give
more cats to the overlappig part.

In theory you could move the relations to a database as suggested Trevor,
but it would be difficult and not efficient to reflect any operations
on geometry
in database. It is very easy to assign 2 cats to the overlapping parts
from 2 input areas, to do that in database (from GRASS) is much more complex.
If you want, you have to move to database everything, i.e. also geometry.

Radim