COMPARISON BETWEEN PHOTOGRAMMETRICAL SURVEYS OF THE ST.STEFANO CHURCH'S CLOISTER IN
BOLOGNA AND METRICS EVALUATIONS OF THE OPERATIONS OF RESTORATION
A. Capra, S. Gandolfi, G. Lombardi™, A. Zanutta
DISTART, University of Bologna
Viale Risorgimento 2 - 40136 Bologna - Italy
KEY WORD: photogrammetric survey, stereo-restitution, Digital Surface Model, Analytical and digital systems
ABSTRACT:
In 1992 a high precision photogrammetric surveying of the monumental complex of S. Stephen Church (Bologna, Italy)
has been realised, in order to perform a set of test on several photogrammetric cameras.
After the restoration, a subsequent survey of the same object has been performed to characterise the changes occurred
due to the human interventions.
In this work, comparison methods of the two independent surveys and the elaborates produced by the analytical and
digital restitution of the object are shown.
Introduction
In 1992 a high precision topographic network on the
northern facade of the S. Stefano Church's cloister has
been performed in order to carry out tests on several
photogrammetric cameras and photogrammetric
restitution systems [1][2][3].
After the restoration work (completed in 1998), a
subsequent survey of the same cloister facade has been
performed. The goal of the work was the detection of
metric changes among the two independents surveys of
the same object realized in different time, before and after
the restoration work. The work was divided into different
phases, topographical survey of control points and
photogrammetric survey of the facade.
The photogrammetric survey has been realized acquiring
stereoscopic photographs using a Veroplast Galileo
metric camera. Using both analytical and digital methods
the Stereo-restitution has been performed. By means
DIGICART 40 analytical stereoplotter Digital Surface
Model (DSM) some horizontal and vertical profiles has
been realized. By means a StereoView 300 Digital
photogrammetric workstation a DSM Raster in sampled
areas was carried out.
The new survey of the cloister
245 points formed the first topographic survey performed
in the 1992, 125 on the northern facade (figure 1). Starting
from the 4 stations put at the first floor of the cloister, the
space resection method was used. The network
adjustment of measurements showed that the semi-axis
of the error ellipses with a 95 % confidence level are
within 2 mm.
In 1998 after the restoration work of the cloister, new
control points and instrumental vertices were realized,
because the oldest had been destroyed or covered. A
new set of 30 points put on the facade were set up and
surveyed from 4 instrumental stations. The scheme and
the realization of the network were quite the same of the
1992.
From each one station, using WILD TC2000 total station
angles and distances were measured.
Figure n. 2 shows the error ellipses obtained from least
squares adjustment at 95% confidence level.
Concern the photogrammetric survey, in order to ensure
full stereoscopic coverage 3 normal photos with a
distance from the facade of about 16 meters has been
done. As above mentioned, a Veroplast Galileo metric
camera with a 151.93-mm lens was used for the
architectonic survey. 13cm x 18cm stereoscopic shots on
glass plates were used.
With a DIGICART 40 analytic plotter 4 horizontal profiles
at different heights and 4 vertical profiles with a 1-
centimeter sampling space (figure 3) have been done.
Residues after the absolute orientation were less than 2
mm in the three coordinates.
Afterwards, to compare the two independent survey of the
same object, we have measured with the stereoplotter,
the tridimensional coordinates of 14 natural points
considered unvaried, distributed along the building hinge
of the facade.
Each natural point has been described with detailed photo
and scheme to make it traceable in both the surveys. The
two groups of coordinates concerning the same points
have been compared through an affine transformation.
So, the 1998 survey was reduced in the 1992 system.
RMS of transformation is very low: 2.8 millimeters.
