[GRASS-user] Newbie questions about GRASS data model

[Moritz Lennert]

Le Tue, 22 Mar 2016 16:58:24 +0100,
"Uwe Fischer" <gisfisch at t-online.de> a écrit :

> Hello list,
> 
> hello Anna, Helmut, Moritz, and Aldo
> 
> I'd like to say thanks for the helpful tips on my questions
> concerning the data model. Especially Aldo's explanations helped me
> such a lot, thank you once more. I appreciate it very much. It was
> possible for me to get deeper into the backgrounds by taking a look
> at the ascii representation of the GRASS data structure and by
> studying Aldo's figures.
> 
>  
> 
> ASCII export makes layer structure, categories and element types
> visible and thus understandable.
> 
>  
> 
> But I still think that the layer concept is nothing unique or special
> in this GRASS data model. If you "rename" the layers (that is: set
> layer1=column1 and layer2=column2) and put the columns in a shapefile
> (for example), then you have exactly the same chances to relate shape
> data to different external files via the different columns, while the
> geometric feature is digitized only one time. Or am I wrong? I know
> that shapes are different because they hold only one type at a time,
> but that don't matter here.

Just to make sure that this is clear: different layers do not
duplicate the same geometric feature. Categories in layers are like
attributes of features, but they are directly linked to the geometry,
thus avoiding the overheads of attribute table handling, and not all
features have to have categories in all layers. You can have features
with no categories in no layer, features with one category in one
layer, features with several categories in the same layer and features
with several categories in different layers.

Moritz