128 
Code 0 
Code 1 
Code 2 
Code 3 
Code 4 
Code 5 
Code 6 
Code 7 
Code 8 
Code 9 
Code 10 
Fig. 2 - Source of the Italian DTM 7.5V10” 
But, as the information layer corresponding to the attribute 
“source code” is coherent, it may be subdivided in regions 
and we can store the same information (as the previous 
one) in a vector region layer, by avoiding storage 
overheads. 
Now the problem we have to deal with, if we use a 
traditional RDBMS, is that each region (as an example, in 
Figure 3 we give the description of the region 
corresponding to the source code equal to 6) has a 
specific shape corresponding in general to a polygon, 
which can be described from the geometrical point of view 
by its nodes and its vertices. 
Note that in the following schema for the sake of simplicity 
we are disregarding more complicated considerations 
linked to the representation of complex features or to the 
topological description of geographical objects, 
in GRASS the vector data are represented by the arcs- 
nodes method. 
The arcs are curves which do not intersect each other; the 
nodes are the extreme points of the arcs. 
The vertices are points belonging to the arcs which are 
not intersection points; they are necessary to draw the 
figure or to compute geometric variables, such as the 
surface or the length of the borderline of the polygon, but 
they do not have topological meaning. 
The arcs-nodes model allow to the description of both 
lines and areas features. 
There are some basic ruies that must be applied to vector 
data: 
• arcs cannot intersect; 
• arcs with a common node, must terminate exactly in 
these points; 
• borders common to two or more features must 
appear only once; 
• areas features must be closed. 
Fig. 3 - Geometrical vector description of the region 
corresponding to source code equals to 6.