The network, that covers an area of approx. 60 km 2 , stretches 
for around 70 km and is composed of 12 polygons (fig. 2), 10 of 
which (from no. 2 to 11) represent the main network and are 
used to monitor the areas most closely involved in the CNR- 
ENEL project. The network also includes the benchmark of S. 
Giuliano Terme, and is characterized by 18 nodes and by 
intermediate benchmarks situated at average distance of 500 m. 
Figure 2: Levelling network 
The other two polygons (no. 1 and no. 12), with a wide mesh, 
constitute the connection network between the benchmarks of S. 
Giuliano Terme and Migliarino Pisano. 
Figure. 3 Closure errors / unit length 
Errors of polygon closure have been found from data processing 
of the main and connection networks; closure errors are 
calculated per unit length, as shown in the diagram of figure 3. 
Finally, the mean kilometric error was calculated obtaining 
values ranging between 0.2 and 0.8 mm. 
3. ADJUSTMENT 
In adjustment operations, the main network has been kept 
separated from the connection network according to the 
criterion which, in the design of the project, led to the 
distinguishing in the general network of these two networks 
that, while complementary, had different characteristics and 
functions. 
The operational method used is that of indirect observations and 
the results reported in the diagram of figure 4, made it possible 
to obtain for s.q.m. a maximum value of ± 2.4 mm and a mean 
value of ± 1.9 mm.