Cl PA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
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Figure 4. The upper scaffolding level was used for scanning
3.1 Topographic Survey
In order to realise a stereoscopic restitution of the Hermes
statue, it was necessary to define a set of control points with
coordinates inserted in a local reference system. A classical
topographic survey by intersection method was performed using
a Leica TCR307 total station (accuracy of angular
measurements 1” and ±2mm±2ppm in distance). The total
station has also the capability to work in a reflectorless mode
with a degradation in the distance measurements. A traverse of
six points on the ground floor and eight targets evenly
distributed around the walls of the room where the statue is
housed, were measured with a redundant number of readings.
This ground control system was used to determine the reference
system of the control points of the photo stereopairs (described
in §3.2). An arbitrary system of coordinates has been adopted
with the Z-axis having its origin at point 01 and abscissa axis
formed by ground point 01 and wall point 2 (Figure 5). The 3D
least squares adjustment resulted to point accuracy within 3mm
at 95% confidence level. The coordinate system defined with
this procedure gave also the georeference of the 3D model
obtained with the laser scanning data.
Figure 5. Schematic of the control network (not to scale)
3.2 Photogrammetric Data Collection
The photogrammetric images were taken independent of laser
scanning data collection. From each location, photographs were
taken by two cameras:
• An analog semi-metric camera, Hasselbland C/M 500, format
5.5x5.5 cm 2 and focal length c = 50mm. The mean distance
from the object was about 1,60m so that the scale of the
photos should be about 1:30 (final accuracy specifications for
the coordinates of the detail points was 0.8mm<lmm)
• A digital camera, Sony DSC-F707 with 2560x1920 pixels (5
Mpixel with pixel size about 4pm) and zoom capability x5.
The photos were taken always with the minimum zoom, with
c=9.7mm given by the manufacturer, and the output was in a
TIF format. The mean distance from the object was 1.80m, so
that agreement could exist with the imaged scenes of the
analog and digital photos.
The dual photography was performed exclusively for research
purposes, so that a comparison of the results of such a complex
object derived from photos of different type cameras could be
studied. It should be noted that none of the two cameras is
metric and their inner orientation data through a laboratory
calibration are not known.
With each camera, 43 photos were taken in total, which create
22 stereopairs, that is:
• 8 photos, one stereopair for each of the four sides of the base
of the sculpture
• 32 photos for the body of the sculpture. The sculpture was
divided for practical reasons into two parts, lower and upper.
One stereopair (2 photos) was taken for each part, along each
basic direction (forward, backward, left and right) and one
stereopair (2 photos) along each diagonal direction. In total, 2
photos were taken for 8 directions at 2 parts, thus 32 photos.
Figures 6a and 6b show the left and right images of the
stereopair, taken along the direction forward-right diagonal
for the upper part of the statue. The details of the body of
infant Dionyssos, which are not pictured on other stereopairs
emphasise the importance for performing such a multiple
photographic coverage of the body of the sculpture
• 3 photos, in two stereopairs, for the coverage of the statue
from the top (cf. Figure 3).
The most difficult part during the photogrammetric data
collection was the establishment of control points, mainly due
to the sensitivity of the object being captured. There were
several methods tested prior to photogrammetric collection,
with the assumption that any physical contact with the sculpture
was not allowed. The most appropriate method proved to be the
projection through a video projector of a laptop computer
screen showing a predesigned grid of lxlcm 2 that was covering
the surface of the sculpture from each location of the
photographic shooting. Simultaneously, the coordinates of
selected nods of the grid were measured from a pair of stations
that were established close to the sculpture. This method was
nevertheless, rejected by the authorities of the museum. It was
considered that this technique possibly causes damage to the
surface of the marble by producing spots due to continuous
exposure to the projector’s light for the duration of few minutes.
The most efficient method finally implemented, was the
placement of 6-12 predefined targets (square black and white)
on light wooden frames or sticks that were placed adjacent to
the part of the statue to be photographed. There was an effort to
locate the wooden sticks in such a way so that the attached