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New perspectives to save cultural heritage
Altan, M. Orhan

CIP A 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
Within a project supported by the Swiss National Science
Foundation and local cultural authorities, the relief and its
constructor Franz Ludwig Pfyffer are subjects of an inter
disciplinary research. The Institute of Geodesy and Photo-
grammetry of ETH Zurich is involved with following tasks:
• 3D reconstruction of the relief for the documentation of
the cultural heritage.
• Analysis of the relief accuracy for the purpose of research
in the history of cartography - a comparison of the
reconstructed virtual relief with current map information
(digital maps and terrain models). This work also
includes analysis of the old maps of the region which
were created at the end of 18 th century.
• Investigation on Pfyffer’s surveying and model
constructing procedures.
The project objectives require a high quality 3D model of
Pfyffer’s relief in terms of accuracy. The overall height
difference between the reconstructed model and the original
may amount to a maximum of 1 mm. Represented by around
11.5 m in reality, this value approximately corresponds to the
accuracy of available national DTM in alpine regions. Thus, a
comparison of the historical and present terrain models in the
analytical project phase (see Chapter 4) can bring reasonable
3.1 Data acquisition
The relief is situated in a narrow cellar room without natural
light on a table about 65 cm over the ground. These conditions
make the image acquisition as well as control point
measurement rather difficult. From a construction platform
fixed at the height of 185 cm above the relief surface, 87
analogue and 50 digital “aerial” coloured images were taken,
both providing stereo overlap for the whole object (Figures 2
and 3, Table 1). The reason for such a double-block image
acquisition was to make sure that the image quality will match
the expectations because no new acquisition would have been
possible any more. Both cameras were calibrated using a close-
range photogrammetric reference field. To avoid disturbing
specular reflection of the shiny relief surface, spotlights in a
combination with dispersion umbrellas were used for indirect
illumination. In spite of this, particularly the digital images
show up rather big reflection and worse radiometric quality
than their sensitive film-based counterparts.
Figure 2. Image acquisition
The three-dimensional position of 40 marked control points
temporarily placed onto the relief surface was determined by
theodolite measurements in a local coordinate system with an
accuracy of 0.09 mm. Considering the unfavourable space
conditions leading to flat and wide-angled rays this result is
satisfactory and sufficient for further processing.
Figure 3. A part of an analogue image (scanned with 1270 dpi)
Rollei 6006
Calibrated focal
83.557 mm
28.871 mm
Number of images
Number of strips /
number of images per
+35 additional
images in
Image size
6x6 cm 2
3060 x 2036 pixel
Scale / Pixel footprint
0.6 mm
Table 1. Parameters of acquired images
3.2 Photogrammetric processing and visualization
Within the previously finished pilot project the functionality of
the automated DTM generation of several commercial digital
photogrammetric stations (SocetSet, VirtuoZo, Image Station,
Match-T) was tested. A comparison of the results with the
manually measured reference showed that the automatic
procedures do not bring required performance in this case. The
height differences between automatically derived models and a
manually measured reference rise up to 5-7 mm in a data set
with a maximal elevation range of about 300 cm. However,
using manual measurements, the theoretical accuracy of 0.3 mm
(0.15%o of the flight height above the object) can be expected.
Main reasons for the problems of commercial image matchers
are distinct height differences and the low contrast in the relief.
As mentioned before, a very precise DTM is needed (height
error of 1 mm in maximum). Thus, the analogue images were
selected as a basis for the photogrammetric processing and we
distorted to manual measurements in the main project phase.
Phototriangulation. The whole block was triangulated
manually on an analytical plotter. The achieved accuracy of
7.62 microns corresponding to 0.17 mm in object space is good
DTM generation. For the DTM generation 48 stereo pairs
along the four strips as well as several additional image pairs in
the mountainous area were measured manually. We used profile