[GRASS-user] Newbie questions about GRASS data model

[Vaclav Petras]

On Wed, Mar 23, 2016 at 10:15 AM, Moritz Lennert <
mlennert at club.worldonline.be> wrote:

> > 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.


You can also say that saying "in layer" is a bit misleading. The geometry
(feature) is not in a layer, rather there is one or more layers of
additional information on top of the geometry.

There are different application of the layer+category system. In some
special (!) cases, layer+category can indeed replace attribute table. If it
is a good practice or not is a matter of ongoing discussion:

https://lists.osgeo.org/pipermail/grass-dev/2015-September/076552.html

Uwe, once you get things sorted out, feel free to edit these wiki pages:

https://grasswiki.osgeo.org/wiki/GRASS_GIS_for_ArcGIS_users
https://grasswiki.osgeo.org/wiki/Terminology_comparison_between_ArcGIS_and_GRASS_GIS

Vaclav
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