different nature, not correlated with the global dynamics of 
the area (point instability, surface ground settling, etc.). 
From the comparison of data recorded during the 
measurements of January and September 1997 (only 
along the coast line) with those ones of February 1998, it 
is possible to point out that in a temporal interval of 13 
months, the monthly mean velocity of subsidence is a 
constant value of about - 4.3 ± 0.4 mm/month. 
With regard to tiltmetry, the scene obtained by data 
processing confirms the subsidence phase interesting the 
phlegrean area. 
It is quite promising the comparison between the results 
obtained by INSAR data processing and spirit leveling 
data. Really, such a comparison has been carried out 
between data set referring to periods lightly different 
(height variations between March 1995 and March 1996 
spirit leveling measurements with an August 1995/August 
1996 interferogram), depending on the possibility to 
choose, for the interferometric data, a more favourable 
baseline, of only 11 m of length. 
A linear extrapolation of the height values of the 
benchmarks at the period of August 1995 and August 
1996, will allow a more significant comparison between 
the results of the two different techniques. 
For this particular case, the first step has been an 
appropriate visualisation of spirit leveling data, which 
should permit to discriminate the areas with higher vertical 
ground displacements from those ones with lower or no 
displacements. Such a result is shown in fig. 12, where the 
height differences between the above mentioned spirit 
leveling measurements are reported. As expected, it is 
possible to note how the area of maximum vertical 
displacement corresponds to the center of Pozzuoli, with 
an entity of the ground vertical displacements radially 
decreasing towards the borders of the city. Such a trend 
shows a good agreement with the results obtained from 
the interferogram of fig 5. 
1*36* 1*40“ 1*44' 1" 48' 
Long., E of Rome, Monte Mario 
40 s 54' 
40* 50' 
-60 -50 -40 -30 -20 -10 0 10 
A h (mm) between 3/96 and 3/95 spirit leveling measurements at Phlegrean Fields 
Fig. 12 Height variations from spirit leveling measurements to be compared with the interferogram of fig.5 
4.2 The island of Ischia 
As in the case of the Phlegrean Fields, also for the island 
of Ischia the spirit leveling is, among the surveying 
techniques used by the Osservatorio Vesuviano, that one 
which is used since more time (more than 20 years). Such 
measurements have thus allowed to evaluate the 
temporal trend of the deformation field acting in the island, 
which results extremely complex. In fig. 13 it is possible to 
point out the most interesting results (reference period: 
May 1990/May 1997), where a subsidence interesting the 
NW sector of the island is shown. The maximum value of 
ground subsidence, pointed out in seven years for the 
benchmark n. 100, is - 9.4 cm (Forio d'lschia). In this 
case, the extrapolated subsidence value is more than 1 
cm/year. 
This is probably due to a gravitational movement of the 
detrital layer covering the volcano-tectonic structure of Mt. 
Epomeo green tuff. The results of spirit leveling 
measurements have thus suggested, as mentioned, the