Moritz Lennert writes:
On 17/06/09 14:58, Roger Miller wrote:
The "layer=" feature of the commands seems to me to be largely if not
entirely superfluous. The function that it performs can be duplicated
by "where="
This would mean mixing different types of objects into the same layer and
thus the same attribute table. I.e. in the field boundaries = roads
example, you would have to use the same table for the centroids
characterising the fields and for the boundary line which (also)
represents a road.
Not very clean in my understanding...
and/or "catlist=".
But this supposes that the user knows the cats, which often is not the
case, and it eliminates the possibility of linking different types of
objects to different attribute tables.
So, neither of these would really replace the current layer feature.
So to continue the usage discussion...
I agree that the first case would not be very clean. The problem in the
example is that it uses one data set for two different purposes, which I
would generally discourage; it will usually lead to other problems. If I
understand the example correctly, then you might get the effect you need by
drawing the boundaries and the areas with different d.vect commands, using
the "type=" capability.
In the second case, GRASS provides several ways for the user to find the
cats. I agree, it would be impossible to associate one vector "file" with
more than one attribute table without the "layer=" capability. Again, that
is a practice I discourage because of the data management problems it is
likely to create. I would merge those attribute tables or I would copy the
vector "file" to a second file and associate the second file with the second
attribute table.
GRASS offers a lot of ways to generate visual layers in a map without using
the "layer=" capability. Keeping the capability gives users more options,
so I wouldn't argue that it should be removed, just that it should be
renamed. I'm not a teacher, but it seems to me to be (in addition to the
problem with the term overlapping in applications) a problem with clarity.
Are students going to understand that visual layers can be created with i.e.
"where=" and that they don't have to use "layer=" to get the layered visual
they need?
If we are to use the term "layer" it would be better to restrict its usage
to refer to a visual object, and to use a different term to refer to
database features that may or may not be used to generate the visual.
Roger
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