The survey methodology must garantee the
possibility to define with high accuracy (0.1=0.2
mm) the D.E.M. of also large dimension objects as
architraves, sculptoral groups, alto-relieves. A non-
contact technique must be used, considering the
marble artworks brittleness.
For all these reasons the more interesting survey
method seems to be the photogrammetric one.
Therefore digital photogrammetry sounds the
technique of the future, due to the quick evolution of
digital cameras and the availability of always more
powerful computers at low cost.
At the moment the scanner-laser technique is
usually applied to survey small bas-relieves;
unfortunately this method becomes unusable for full
relief and large dimensions sculptures. Infact in
these cases the D.E.M. generation has to be
divided in different phases, and the laser technique
cannot define tie points indispensable to joint the
several D.E.M.s [Scaioni, 1995].
The “digital photogrammetric measurement system
InduSCAN" by Carl Zeiss Jena GmbH (Germany)
seems to be the more suitable tool to solve the
survey problems before described [Kludas, 1995].
The automatic copy realisation from D.E.M. can be
nowadays carried on using of the high technology
level of the N.C. milling machines. Big problems are
still present to define the correct path of the N.C.
milling machine tool and for the memory dimension
of their controller (usually used with smaller set of
data) [Monti, 1994].
In the next paragraphs the different items will be
described. The methodology exposure is carried out
describing the theory and its application (survey and
physical reproduction) at two marble elements
coming from the Duomo.
3. SURVEY AND D.E.M. GENERATION WITH
InduSCAN SYSTEM
The survey of the sculptures has been realized
using “InduSCAN system” by Carl Zeiss Jena,
usually employed in high precision 3-D industrial
measurements. The task was to obtain a 3-D
D.E.M. of these objects [Claus, 1988].
Ahead to begin the technical description of the
different survey phases, it is necessary to define the
accuracy level and the final quality. The Cathedral
sculptures are nowadays copied with a traditional
methodology: a sculptor makes the statue observing
the original. The aid of the modem technology is
restricted to the application of pneumatic rock-drills
to work the marble and milling machines for the
blocks rough-shaping. The reproduction accuracy is
strictly committed to the sculptors, which use
suitable three-dimensional pantographs to report the
original measures on the copy.
For all these reasons a survey accuracy of o=+1
mm in X, Y, Z directions can be considered as
enough. However in the tests later described we
have looked for the best accuracy, so to check and
validate the whole methodology.
Fig. 1 - Image acquisition with InduSCAN
3.1 The sculptures survey
Two sculptoral elements, very representative of the
artwoks typologies present in Milano Cathedral,
have been surveyed:
e a statue (dimension 50x50x160 cm);
e a little spire (dimension 40x40x140 cm).
These two handmades have been carried from the
Cathedral to "Cantiere Marmisti^ , where a room
has been prepared for the survey with InduSCAN
(see figures 1 and 2).
The elements to survey have been placed on a 50
cm height bearing, in the way to permit some
takens from down toward up. The room was
darkened to allow the statue lighting with a
projector.
The pieces have been signalized with some retro-
reflective targets, later used for the image
orientation.
The targets have two main tasks for their use in the
bundle adjustement process:
"A workshop in Milano where usually the marble copies are
created.
582
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B5. Vienna 1996
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