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. > > 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. Moritz Lennert wrote: > 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: eXactly that. Milles mercis, Nikos _______________________________________________ grass-user mailing list [email protected] http://lists.osgeo.org/mailman/listinfo/grass-user
