DIGITAL ORTOPHOTO OF SAN MARCO’S BASILICA IN VENICE
G. Auditore‘ D. Miniutti”L. Pilot”
"DIIAR -Politecnico di Milano
"CIRCE - Istituto Universitario di Architettura di Venezia
auditore@cidoc.iuav.it
miniutti@cidoc.iuav.it
pilot@iuav.it
KEY WORDS: mosaic floor, ortophoto, distorsion parameters, distorsion correction, photogrammetric blocks
ABSTRACT:
The office for the conservation of the San Marco's basilica in Venice asked to the Politecnico of Milan to realize the survey and the
digital ortophoto of the church's mosaic floor. This kind of work will guide and help all the people interested in the conservation
process.
The complete ortophoto's realization created some problems depending to the object's dimensions , also considering the plot's scale.
The digital camera employed for the photogrammetric blocks was the Rollei d7 metric camera. Some tests have been done to
evaluate distortion parameters of this camera: The results show that this camera presents some elevate values of distortion, about 60
microns, not really comparable with a metric camera.
It was impossible to use softwares that doesn't accept, in interior orientation, images with distortion values larger then metric
cameras. So it was realized a software that allows, from the original image and loading the calibration card, to resample the digital
photo, geometrically correcting the distortion.
In this way we have images teorically without distortion, so that they can be used with any digital photogrammetry software for the
ortophoto's production.
To test the images corrected from the distortion it was used the Apex software produced by PCI Geomatics to orientate the
photogrammetric blocks of the original and corrected photos. At the end of this process we compared the different residual of the
two blocks.
It was realized two different projects having each one a different calibration card, the first with the distortion parameters reported
from the Rollei Factory , the second with no distortion values.
The orientation process obtained by aereal triangulation, collimating the same control points and tie points, produced comparable
values for the two blocks.
The paper treat widely about the distortion correction software and the photogrammetric methods to obtain digital ortophotos.
The need and the urgency to produce a valid instrument to
support the preservation and restoration work on the museum
floor in the Basilica Marciana has led to the Procutaroria of San
Marco to undertake creation of a precise and accurate
orthophoto of all the parts of the mosaic flooring (Figure 1) in
the church, both in the internal portions as in the external
narthex.
Design of the work has therefore had to consider the multiple
needs of those involved in preservation, translating the
instructions given by the conservation methodologies into a
comprehensible definition of appropriate representations and
processing of the mosaic.
Considering these factors, the nominal scale of the work was set
at 1:5 so that the resolution of the orthophoto would allow
identification of each individual tile; another need considered in
the design phases was the possibility of printing the final
product at a 1:1 scale, so that those involved in restoration
would be able to use the support in order to temporarily position
the tiles removed from the floor before undergoing treatment.
A final need in the project was the ability allowed to the final
user to extract the vectorial sections from the orthophotos that,
appearing at a real scale in the form of wooden templates,
would guarantee reconstruction of the geometric structure of the
floor after it was disassembled.
Definition of these categories of action has subsequently guided
us in the choice of hardware and software solutions for
realization of the job; since the total mosaic-covered floor to
cover with photograms is 3000 m2, we chose a Rollei D7 metric
(Figure 2) camera for the job, a digital camera that allows to
streamline the phases of the shots and processing the
photograms.
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