t is 
of 
as 
ting 
the 
the 
have 
to 
ence 
lace 
rth- 
one 
ini, 
was 
to 
as 
ty's 
ment 
hile 
the 
hile 
akes 
is 
iano 
hich 
er's 
able 
t in 
in 
ande 
rona 
ely. 
= by 
859. 
ques 
rate 
test 
phy, 
ring 
gram 
tion 
tti, 
, and 
  
The degree of incline is only one aspect of the 
settlement that the Tower has experienced and a 
complete survey will clearly include a variety of 
elements and a thorough knowledge of three- 
dimensional geometry is of primary importance to 
any such research. Two surveys of the entire 
structure have been undertaken to date, one in 
1908 by Bernieri and the other in 1965, under the 
leadership of Polvani (published in 1913 and 1971 
respectively). These surveys were incomplete for 
three main reasons: 
i) they used direct measurements and were, by 
definition, limited to points that were 
physically accessible to the surveyors 
ii) in such circumstances, a choice of measuring 
points cannot be planned and the correlation 
between the points is necessarily random, 
while the difficulty of access makes 
measurement difficult and can lead to 
cumulative errors 
iii) using direct measurements on a monument that 
is made up of curved forms and a variety of 
angles of inclination, it is almost 
impossible to represent the Tower within a 
system of spatial coordinates that might be 
sufficiently accurate for use in possible 
interventions in the future. 
If we then consider the fact that the Tower's 
shifts did, in the past, alter direction then it 
becomes clear that a knowledge of three- 
dimensional geometry is insufficient by itself. In 
order to ascertain the behaviors of the Tower 
during the two centuries of its construction, it 
is necessary to conduct a detailed survey of each 
of the building blocks, storey by storey. 
For the reasons stated above, it is clear that 
only a photogrammetric survey would be sufficient 
to provide the information to the degree of 
accuracy required. 
A preliminary terrestrial photogrammetric survey 
was undertaken in 1983. The data gleaned from that 
partial and experimental survey provided much more 
information than expected and studies are yet to 
be completed (Baj and Bozzolato, 1988). 
In agreement with the Opera della Primaziale it 
was decided to undertake a full-scale 
photogrammetric survey to cover the entire Tower. 
3. SETTING UP THE 1990 PHOTOGRAMMETRIC SURVEY 
This time around, the AGIP team had at its 
disposal a helicopter-mounted SER system which was 
specifically-designed for use in difficult 
circumstances and has been mounted on an Agusta 
AB-412  (Bozzolato, 1989). This equipment would 
have made it possible to provide synchronized 
stereo images of the entire Tower from the 
vertical, the horizontal, and from a variety of 
angles down to a distance of twenty-five meters. 
This proved impossible, however, as the government 
had, in the meantime banned over-flying of the 
Tower for fear that the added vibrations might 
further destabilize the structure. 
The team therefore was forced to adopt a simpler 
approach which was an aerial platform mounted on 
the back of a truck. 
   
T A FA 
s Y Q 
s. Fie 
     
    
  
    
  
    
   
  
  
  
  
  
    
   
  
   
  
    
   
    
   
  
  
   
  
   
  
  
  
   
   
   
  
  
  
  
  
  
   
  
  
   
    
     
   
   
  
   
  
   
  
   
    
  
   
     
   
  
  
  
  
  
  
   
   
      
   
   
  
   
  
     
During the planning stages it was also decided to 
mark out the points from which the photographs 
would be taken, as well as to refurbish the 
photogrammetric control points used in 1983 and to 
add some new ones. The spatial coordinates of all 
the points were also to be determined on the basis 
of a new triangulation network which would relate 
to the Italian Cartographical System (Sistema 
Cartografico Nazionale). 
4. THE FIELD WORK 
4.1 Pre-targeting 
During the preliminary reconnaissance work, we 
noted that a number of geognostic and geophysical 
surveys, as well as maintenance work to 
underground structures over the intervening five 
years had destroyed a number of the stations of 
the triangulation network and the photogrammetric 
control points marked out in 1983 on the paving 
below the Tower and on the grass adjacent to it. 
Apart from a few bolts that had been cemented into 
the paving, the only undamaged control points were 
the nails that had been hammered into the masonry 
of the Tower. 
In order to achieve the accuracy required by the 
project, it was necessary to map out a new, more 
precise triangulation network (Figure 1). 
MURA URDANE 
  
  
  
  
  
  
|l 
SE 
C 
[ 
= 
^ 
7 
5 
Ex ol & 
By: 
b 
TI 
5. 
Az 
> 
  
  
a LZ i 
NN mh 
uli x L zl. Ik 
7 i£ PR À, | 
7 CN | P 
Figure 1 
Piazza dei Miracoli, topographic network, 1990 
Four concrete-capped pillars were constructed 
where triangulation stations were fixed. Three 
other stations were fixed on the parapet of a 
loggia of the Duomo Museum. All the stations were 
marked with cemented-in bolts. Three stations were 
placed on a special stabilized tripod. 
The twenty-four 4 x 40 millimeter steel nails 
(control points) fixed into the interstices of the 
masonry blocks of the Tower in 1983 were 
supplemented with a further thirty-two. All of the 
fifty-six points were marked with a black "Malta 
cross" on a white ground as well as a number, so 
as to make them clearly visible from the 
triangulation stations as well as on the images 
produced. 
The twelve bolts on the ground that had survived 
following the 1983 experiment were highlighted 
with paint and were used as supernumerary controls 
between the earlier findings and the 1990 survey.