[GRASS-user] Newbie questions about GRASS data model details

[Helmut Kudrnovsky]

some of my naive understanding of the data model:

>First question: how are the different type determinations for geometry
objects saved in GRASS? What >makes a boundary a boundary and separates it
from a line? The same for points and centroids.

taken from https://grass.osgeo.org/grass70/manuals/vectorintro.html

- point: a point;
- line: a directed sequence of connected vertices with two endpoints called
nodes;
- boundary: the border line to describe an area;
- centroid: a point within a closed ring of boundaries;
- area: the topological composition of a closed ring of boundaries and a
centroid; 

an area is build upon boundaries and centroid; e.g. change line to
boundaries and point to centroid and build and area

>Second: Up to now, I was not able to understand the concept of layers in
GRASS. I've read a lot of >explanations, most of them telling that a layer
is a set of category values making up a different view to >the underlying
geometry objects. Using layers, the geometries can be aggregated or
individually marked in >a different, yet parallel manner at the same time.
>
>But everything described in the manual or Wiki seems to me like one can do
this using different attribute >columns as well. 

just think of a line representing a road: e.g. layer 1 represents the road
type (e.g. small road ... highway), layer 2 holds the information if it's
allowed for bicycles; with the layer concept you doesn't need an attribute
table (but you can have also attribute tables in every layer) to hold
different characteristics of e.g. a line, boundary, point etc.

but obviously there is some overlapping between the layer and/or attribute
concept...




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best regards
Helmut
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