On 06/06/13 14:52, Nikos Alexandris wrote:
Nikos A wrote:
Still, I find it counter-intuitive that it's required to create
a new
vector map to achieve that.
--%<---
v.category in=wrs2_tiles_of_interest_testing@PERMANENT layer=2
option=add
ERROR: Output vector wasn't entered
--->%--
Markus Metz wrote:
Because you need to modify vector geometries in order to add a
new
layer. Categories and layers are first and foremost stored
together
with the geometries. Whenever geometries are modified, the result
will
be a new vector map (with the exception of v.edit and the
digitizers).
Nikos A:
Just for (even more) completeness and understanding: adding a new
layer
does not really change the geometries per-se (that is the
topology, the
shape).
Markus Metz:
No, it does change the geometry directly.
O-K. This is where I am confused.
It's the inner design of the vector data model, i.e. the fact that
categories and layers are bound together. Is it wrong to describe
it as an
internal "indexing" issue or similar?
Yes, it is wrong. Categories and layers are first and foremost
stored
together with the geometries.
I'll try to consult the programmer's manual or else about the vector
data model. I need to clarify this to myself :-).
Therefore adding a new layer will modify the geometries directly.
IOW,
categories and layers are part of the (GRASS) vector geometry
objects.
Rigth, I need to (re-)define to myself the "geometries" term.
Also, layers and categories have nothing to do with topology.
A-ha...
A new layer really is only added by modifying the geometries, only
after
that you might want to attach a table to that new layer.
Will check the manuals...
I guess you confuse layer with database connection. The existence of
a
layer is a condition for an optional database connection. A database
connection itself does not mean the layer of this connection
actually
exists.
Danke Markus.
I am not really confused over the layer/db-connection stuff. Rather,
it is the topology - geometry - categories and layers stuff that I am
confused about.
I think the confusion comes from the fact that what Nikos calls
"geometry" is the part "drawn" on the screen, i.e. the geometrical
features visible to the user when displaying a map. Markus speaks about
"geometry" in the programming sense, i.e. the programmed structure that
represents the geometrical features, but which contains more than just
the "visible" geometric parts (i.e. the coordinates), e.g. it also
contains the information about the type of geometry and the possible
layers (aka fields) and category values. See:
http://grass.osgeo.org/programming7/vlibFormat.html#vlibCoorFileFormat
Moritz
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