ANALYTICAL PHOTOGRAMMETRIC SURVEY OF THE LEANING TOWER OF PISA
Baj E., Associate Professor, University of Milan, Italy
Rampolli M., Offshore Standardization Manager, AGIP, Milan, Italy
Bozzolato G., Consultant, AGIP SpA, Milan, Italy
Commission V
The University of Milan and AGIP SpA repeated the photogrammetric survey of the Leaning
Tower of Pisa. The paper relates the procedure followed and the results obtained. The
exposures were taken from an elevator at a height of about twenty-three meters from the
ground such that all the orders which make up the Tower and the Bell Chamber were clearly
visible in the photograms. Sixty-six exposures were taken in all, some in color and some
in black and white. The color ones were employed for plotting after their reliability was
tested. The aim, as in the previous survey was to determine the shape and the spatial
position of the successive orders of the Tower and of the dripstones which limit them. An
analytical plotter was employed to determine the coordinates of the points. The collected
data were processed using a CAD station and appropriate software in order to solve the
geometric problems and to display the results. Finally, the results of the previous survey
and the present ones were compared.
Key words: analytical, close-range, photogrammetry, terrestrial
1. INTRODUCTION
The Torre Campanaria della Primaziale di Pisa, or
Bell Tower of Pisa cathedral, which we shall
simply call the Tower from now on, is world-
renowned not only for its artistic beauty but also
for its extraordinary "lean". Much has been
written about the Tower, which was started in 1174
and was completed shortly after 1370, and details
of the dates in its construction are included in
an earlier work by the present authors (Baj E. and
Bozzolato G., 1989 .
To help set in context the purpose of the second,
and more exhaustive photogrammetric survey of the
exterior of the Tower of Pisa, we feel it useful
to concentrate on the construction and control
techniques used by the builders of the Tower. Our
aim is to show how the way the Tower was built
created the very premise for using photogrammetry
to achieve our aim. This technique is in fact one
of the few available that can reproduce a
geometric configuration as well as provide data
that may be integrated to other information
collected by means of direct measurements.
2. BACKGROUND TO THE CONSTRUCTION AND CONTROLS
At the time the Tower was built, the technique of
leveling had not yet been developed (and was
actually first described by M. Thevenot in 1666,
that is some four hundred years later). At the
time, the constructors had at their disposal only
the plumb-line and other simple devices.
Rudimentary as they were, these instruments were
able to provide accuracy of the horizontal to
within + one centimeter across the entire eleven-
to fifteen-metre width of the Tower.
The unstable nature of the ground in the city of
Pisa was considered a typical feature of the area.
The frail upper surface initially sank under the
weight of a heavy body but, over a period of time,
increased its consistency and resistance.
It was common practice for a stock of elements,
such as the hewn blocks of stone, the columns, and
the arches to be prepared before the building work
was to start. This enabled the marble workers and
the masons to keep pace with the rest of the
building processes. This was particularly so in
the case of modular structures such as the Tower
of Pisa. Given the instability of the land, it is
likely that the master builder of the Tower of
Pisa arranged for each "storey" to be put up as
quickly as possible and for there to be a "waiting
period" while the ground settled, after which the
next "storey" could be started.
Measurements taken on certain exposed areas of the
stone at different heights of the Tower have
revealed that the stones were trimmed to
accommodate settlement. This shows that subsidence
in more than one direction must have taken place
during the construction of the Tower, the north-
south shift being the most evident and the one
that took precedence after completion (Lumini,
1967).
While it is clear that this subsidence was
noticeable, the phenomenon would not appeared to
have concerned the architects overmuch, used as
they were to the fact that many of the city's
buildings were not quite perpendicular. A document
published in 1298, in other words while
construction was still underway discusses the
first checks on the trueness of the Tower. While
describing the approach used in detail, it makes
no reference to any settlement. One thing is
certain, though. In about 1384, Antonio Veneziano
painted a fresco of the Camposanto in Pisa which
shows the completed Tower leaning.
Over the years, the issue of the Tower's
propensity to lean has attracted considerable
attention. Among others, Vasari wrote about it in
1562 while De La Condamine examined the issue in
1755 and Soufflot three years later. De La Lande
returned to the subject in 1790 as did Da Morrona
and Gherardesca in 1812 and 1838 respectively.
Relatively reliable measurements were taken by
Cresy and Taylor in 1817 and by De Fleury in 1859.
It was only this century, though, that techniques
and instruments that might provide more accurate
readings were developed. Using the latest
techniques, the Institute of Geodesy. Topography,
and Photogrammetry in the Faculty of Engineering
of the University of Pisa set in train a program
of annual testing of the degree of inclination
based on specific points on the Tower (Cicconetti,
1927, 1931, 1948; Ballarin, 1953, 1959; Geri and
Palla, 1988).
$4. 8 6 9. MA MN 28 3
