Source: MCAE Systems s.r.o.
In 1999 the State
institute for the Care of Historical Monuments in Prague with
the support of the Czech Ministry of Culture began a long-term project,
monitoring the Charles Bridge sculptures. The Charles Bridge, dating
from the 14th century, is not only a significant historical monument,
but also a gallery of baroque plastics.
The sculptures and sculpture groups are affected by degradation
and the original plastics were therefore gradually replaced by copies.
The research on the originals and its copies can derive now information
about the actual condition of the sculptures and the damaging influence
of various environment factors. For the monitoring of the actual
situation, non-destructive measuring methods, including a detailed
photo documentation was used. To actually get the 3D form, the "flat
pictures" can not give enough information. Therefore an accurate
digitizing was needed, producing data which can easily be archived,
compared and used for making a replica if needed.
The sculpture of Saint Vojtech was the first element to be digitized.
The original, made from sandstone by the most famous sculptor Ferdinand
Maxmilian Brokoff, is dated from the 18th century and is 6 m high.
The digitizing was made by MCAE
Systems s.r.o. using the optical measuring systems TRITOP and
ATOS from GOM. TRITOP is a photogrammetry
system which measures the exact position of markers. The system
is portable and consists of a high resolution digital camera, a
scale bar and some coded markers and a notebook with the evaluation
software. The markers are placed on the object and some coded markers
and one or two scale bars are added. Then images are taken with
the digital camera from different positions. These images are downloaded
into the notebook and then the evaluation software defines the exact
3D position of the center of the markers on the object.
The optical digitizer ATOS is portable and works on the principle
of projecting light fringes on the object. Each projected image
is viewed by the two CCD cameras built in the ATOS sensor. Then,
from the images, a cloud of points, which exactly defines the surface
shape of the measured object is calculated. Depending on the resolution
of the CCD cameras in the ATOS digitizer, 400 000 up to 1,3 mil
object points are captured and measured in one measurement. The
clouds of points are then registered into the predefined grid of
marker points defined by TRITOP. From all the measurements a complete
information about the whole surface of the measured object is captured.
To get good details, and allow an efficient working, the ATOS digitizer
can be calibrated to a measuring area of 100 by 80 mm and then,
to scan large areas to 350 by 280 mm or bigger.
Combining both systems (TRITOP and ATOS) assures excellent accuracy
and easy integration of the individual digitizing measurement, therefore
both systems are used to scan this sculpture. To be able to handle
the big amount of data, the measurement had to be split into the
front part, the back part and the pedestal, but all data is in the
correct coordinate system.
The sculpture was first cleaned and then the reference markers
are applied, partially using a lift with the cabin as shown in fig.
1. Totally about 1600 marker points were placed on the sculpture
|Fig. 1: Application of the markers
on the monument
|Fig. 2: detailed view of the statue
with the markers
Then TRITOP is used to exactly define the position of all these
markers. Typically 100 views were taken during the day using the
digital camera. From all these pictures, the position of the markers
in the images are defined. Then the images are virtually matched
together to define the actual camera position during recording and
then calculated the exact 3 D position of all markers as given in
|Fig. 3: Marker positions and reconstructed camera positions
as given by the TRITOP photogrammetry measurement. The marker
"grid" is then used to integrate the ATOS scanning
measurements automatically into their position.
The digitizing by the ATOS system was done mainly during the night,
to have low ambient light conditions and good fringe contrast on
the sculpture. For the digitizing, the ATOS sensor was mounted on
a camera crane. This crane was controlled by a joystick and had
a camera built in to verify the working area. Then the ATOS sensor
was positioned in working distance around the sculpture and measurements
were taken as given in fig. 4. . For this sculpture, more than 350
views are digitized by the ATOS digitizer and more than 37 million
data points are gathered. Depending on the sculpture and the data
density needed, the measurement of one sculpture takes typically
2 to 3 days for an experienced team.
|Fig. 4: Set up to digitize the sculpture
From all the measurements taken by the digitizing system, a digital
3D model was established as given in the enclosed figures.