4.2 Geodetic survey
The following approach was adopted:
i) all the angles between the directions to
the triangulation stations were measured.
Given that these were all less than 150
meters apart, three measurements were
considered sufficient
ii) as far as the photogrammetric control points
were concerned, we decided to measure both
the horizontal and the vertical angles twice,
given that they were less than one hundred
meters apart
iii) angles were measured using the WILD T3
theodolite
iv) the distances between the triangulation
stations each way were measured using an AGA
140 geodimeter
v) the difference in height between the
photogrammetric control points was calculated
using trigonometric leveling with reference
to one specific point. The height of this
point was determined by leveling using a
JENOPTIK NI 007 in relation to a benchmark of
the Istituto Geografico Militare (IGM).
Computations and adjustments were made using
Professor Bencini's Planasc program in the
Computing Laboratory of the Geodetic Division at
the IGM. Thus we obtained the plane/altimetric
coordinates of all the triangulation stations and
of all the control points, their standard
deviations as well as error ellipses.
The maximum values of the error ellipses were
found to be + three millimeters. The standard
height deviation for the control points was + 2.7
millimeters. These values are in line with the
degree of accuracy required for this kind of
project.
In order to ensure the best orientation of the
stereograms in the restitution, the height and
position of each photography point was determined
using an electronic theodolite.
4.3 Planning the photography
As stated above, an aerial platform, the so-called
cherry-picker was used. This consists of a basket
on an adjustable arm which is in turn mounted on
the back of a truck. Large enough to hold two
people with their equipment, the basket may be
lifted with an extraordinary degree of accuracy to
the height required. The use of this machinery
offered us a considerable degree of freedom of
movement around the monument on paved areas as
well as up to a height of twenty-three meters.
Given the cylindrical shape of the building, the
twelve photogrammetric stations were positioned
almost equidistant from each other, and thus they
formed a dodecagon. Two sides were missing however
as, in the South-East corner, there is a convent
Which is now the Museum of the Duomo. To
compensate for this, two stations were located
higher up and further back where a paved road made
manoeuvering of the aerial platform possible
(Figure 2).
Each photogrammetric station was thus some thirty-
five meters from the Tower and about twenty-three
meters above ground level. Each side of the
polygon was approximately twenty-seven meters in
length. The camera was placed as near to
horizontal as possible and at right angles to the
sides of the polygon. This meant that two
exposures were taken from each of the stations,
one normal to the previous and one normal to the
next, respectively. A considerable degree of
tolerance had to be allowed because of the
pronounced lean of the Tower and the movement of
the vehicle on which the camera was mounted.
COLONNA ^ 4j
] CON tura i.
Figure 2.
Plan of the Photogrammetric stations
A total of sixty-six photograms were taken. Of
these, forty-four were in black and white on an
15° DIN orthochromatic glass plate. Twenty-two
exposures were taken on color transparencies.
After a preliminary examination, thirty-three
pairs of pictures were selected for possible use
in the next stage of the project.
While the format of the exposures was thirteen by
eighteen centimeters, with the longer side
vertical, the considerable sideways overlap
enabled us to survey the entire exterior of the
Tower using only six pairs of photograms.
The height that could be achieved by means of the
aerial platform enabled us to photograph the
entire Tower (Figure 3) with great flexibility and
an ideal range of control points.
The entire photographic session took just under
two days, with the exposures being developed as
soon as they were taken. Only one camera was used.
While this offered homogeneity in terms of image,
the disadvantage was that synchronization was
clearly impossible. There was thus a slight
variation in shadows in the pairs of pictures.
Figure 4 shows a couple of photograms.
5. RESTITUTION
The Tower of Pisa is made up of eight orders. A
round base, is topped with six arcades with a
central core surmounted by a bell chamber. The
base and bell chamber are decorated with relief
arches while each of the order is circumscribed by
cornices or dripstones.
