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CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
the centre of the modern city.
Grayscale orthoimages of both sides of the whole monument
were previously generated by the Laboratory of
Photogrammetry of the School of Rural and Surveying
Engineering of NTUA, during a research programme funded by
the Hellenic Ministry of Culture (Figure 1). The images had
been acquired using metric cameras such as UMK 13x18, P31,
and Rolleiflex 6006.
For the application needs in this paper, a detailed close range
video recording of the eastern side of the Gate of Adrianos was
made. Finally only a small part of this side was selected for
editing, testing and production of an ortho-photomosaic. That
part is a column capital, shown magnified on the lower part of
Figure 1 (inside the red circle).
The comparison of the orthoimages coming from the two
different image sources, metric cameras and digital camcorder,
and the evaluation of geometrical accuracy and radiometric
quality will be examined.
2. THE DIGITAL CAMCORDER
For the application purposes a Sony DCR-TRV140E (Figure 2)
supporting the Digital8 standard was used. It is one of the
cheapest digital camcorders in the market. The connection with
the computer is achieved through the DV-OUT port supporting
the IEEE 1394 protocol. The effective part of the 14" CCD
provides more or less 350K actual pixels. The focal length
varies from 3.6 mm to 72 mm according to the manufacturer’s
specifications. The zoom has no fixed positions but a
continuous range and it is manually controlled. Thus it is almost
impossible to achieve a known and repeatedly steady focal
length. Hence it was determined that the lens should be fully
retracted in order to have the focal length at its shortest length.
The focus was fixed manually at infinity. A Pinnacle Studio
video card was used in order to create captured clips in AVI
format from the camcorder. The captured videos and the
grabbed still frames were created by using Adobe Premiere.
The video speed e.g. the frame rate was 25 fps and the average
data rate 4,28 MB/sec.
The change of image’s aspect ratio from 1.25 to 1.28 should
also be considered.
3. CALIBRATION OF THE CAMCORDER
The low image resolution in combination with the usual needs
of archaeological applications demand the achievement of
maximum accuracy possible from the photogrammetric
procedures, thus the accurate determination of inner orientation
parameters, i.e. principal distance, principal point coordinates,
radial and possibly tangential distortion parameters. For the
calibration of the particular camcorder, video images of a three
dimensional testfield were taken. The testfield was placed in a
room at the School of Rural and Surveying Engineering of
NTUA, and its dimensions are 6x6x3 m 3 area, consisting of
targets which are attached to a metallic board and on 8 digit
columns (Figure 3). The coordinates of the targets had been
calculated by topographic intersections of high accuracy, better
than 3mm. The testfield was video recorded from several
distances varying from lm to 5m, and from different angles.
The result was a great number of different scale images with
and without overlap, with and without convergence.
The influence of the radial distortion is more than obvious,
looking at the curved appearance of the vertical columns (see
Figure 3). It was expected, since a wide-angle lens was used.
(Devernay et al, 2001; Karras et al, 2001).Twelve images were
chosen and most of the visible targets were measured with pixel
accuracy. Initially the images were inserted in AutoCAD®
environment, and the image coordinates of the targets were
measured. Through an affine transformation the measured
coordinates in ‘screen’ units were transformed in pixel units.
Some of the distant targets appeared fuzzy and they were
rejected. (Figure 4).
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Figure 2. Sony DCR-TRV140E
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The file type of the still images is uncompressed TIFF. The
quantity of 350K pixels is inadequate and leads to an a priori
poor quality. Although the video card provides -during the
frame grabbing process- an image size of 720x576 pixels, part
of the original image is cropped out. Thus the size of all the
images of the present work is 706x550 pixels. The image loss
of 2% and 4.5% at horizontal and vertical direction respectively
is significant not only for the coverage of the recorded object
but also for the overestimation of the computed focal length.
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Figure 4. Detail of distant targets
The determination of camera calibration parameters was made
simultaneously with the calculation of the exterior orientation
elements of each image in a bundle adjustment solution. An in-
