TOTAL SINKINGS IN TIME (1975-1999)
E
& 1,0
0,0
CD
o
CO
o
o
o
CO
benchmarks
<1>
D
TO
Figure 4: s.q.m. of the altitudes of the main network vertices
TIME (years)
Figure 5
4. CONSIDERATIONS ON THE VERTICAL
MOVEMENT OF THE MIGLIARINO PISANO
BENCHMARK
The benchmark IGM 31/8 of Migliarino Pisano is of particular
interest, as it has been taken as reference in the study of the
vertical movements of the ground in the Pisan Plain.
Since this datum is located in alluvial soil, even though on the
side of a big dune, archive and land reconnaissance research led
to the identification in the 70ies of another benchmark firmly
based on rocks: the IGM 15 III C (horizontal) dating back to
1920 and located at the base of the Pisan mountains in S.
Giuliano Terme.
As has been said before, the link between these two benchmarks
has been considered a relative altimetric reference since 1975.
Analysis and the results obtained in the various levelling
campaigns performed on this stretch since 1975 and shown in
Table 1, make it possible to evaluate the trend of negative
vertical of the Migliarino Pisano benchmark in comparison to
the rock benchmark of S. Giuliano Terme.
Periods
Years
t
Vertical
movements
(mm)
H
Time
delays
(years)
At
Variation
of vertical
movements
(mm)
AH
Gradient
of the
vertical
movements
(mm/year)
AH/At
1975
0
0
s
-23
-3,83
1975-1981
e
-23
2
-9
-4,50
1975-1983
8
-32
!l A
-5,00
1975-1984
9
-37
1 01 -12
-1,20
1975-1994
19
-49
5
+ 13
+ 2,60
1975-1999
24
-36
Table n° 1
This data shows that the relative vertical movement of the
benchmark in question remained negative from 1975 to 1999,
i.e. for 23 years, figure 5.
To explain the phenomenon, the trend of AH /At has been
reported in figure 6, clearly showing a variation during the years
81-83. Checks carried out show that the drawing off of ground
water was significantly reduced locally during these years.
Therefore, the progressive raising of the latter may account for
the swelling of the ground that possibly underwent maximum
depression (fig. 5) in about 1990 (the derivative becomes zero
in fact in this year as shown in fig. 6).
GRADIENT OF SINKINGS IN TIME (1975-1999)
Figure 6
Today it would therefore be reasonable to forecast that the
altitude of the I.G.M. benchmark of Migliarino Pisano in
comparison to that of S. Giuliano Terme should settle to a value
of approx. -28 mm (fig. 5) compared to the 1975 initial datum.
These results together with the latter remarks regarding the
general subsidence phenomenon in the Pisan Plain, suggest that
it is essential to repeat the measurements with levelling
campaigns on a regular basis.
Local subsidence, arising above all for anthropical changes of
the territory could be carried out with a GPS method, both in the
well-known and used static modality, with a sub-centrimetric
precision, and in the cinematic modality with a centimetric
precision.
In fact the network is developed in the plain area along roads
with few trees and therefore with a good visibility of the
satellites, and this makes it possible to carry out planoaltimetric
recordings of the trajectory of a vehicle moving at low speed,
equipped with GPS receiver using a continuous kinematics OTF
method, with pauses at the levelling benchmarks; therefore with
a STOP and GO modality.
This methodology has already been used in several cases and in
particular in the altimetric survey of Ronchi dei Legionari
airport strip, figure 7 ([1] G. Manzoni, R. Cefalo, S. Fonzari.
1994). In this study the altitudes of strip axle were determined
and compared with those obtained with a photogrammetric
method (fig. 8). As it was expected, the GPS cinematic method
has not reached the precision of the geometric levelling,
showing an accuracy of approx. 5 cm., which can be improved
using choke-ring antennas and multipath attenuation algorithms
and with a 5 or 10 Hz sampling, so as to produce points density
of very high trajectories.
This method would also allow the monitoring of local sinking
of the road-bed due to very superficial factors, which are not
