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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
measurements at a profile distance of 1 cm with additional
breaklines, which gave us about 300'000 points in total. The
grid calculation with an interpolation software system DTMZ
developed at the Institute resulted to a regular raster of 1 cm
grid width.
Orthoimage generation. The 87 coloured images were
scanned on an UltraScan 5000 scanner of Vexcel Imaging with
the resolution of 1270 dpi corresponding to a footprint of 0.4
mm in the object. This assures that even the smallest relief
details are clearly visible. For orthoimage generation the system
SocetSet of LH Systems was used. The resulted mosaic covers
the whole DTM area and uncompressed has a file size of about
306 MB.
Extraction of 3D vector data. In order to get a vector data set
for the accuracy analysis of Pfyffer’s relief, significant relief
features like roads, rivers, lakes and settlements were captured
three-dimensionally in a manual mode on the analytical plotter.
Texture mapping and visualization. Mapping the orthoimage
onto the DTM, a variety of visualization products were derived
(Figure 4): anaglyph images, interactive VRML models and
flyovers. A virtual flight over the reconstructed relief or an
online navigation in the model is nowadays as fascinating as in
the age of Franz Ludwig Pfyffer the relief itself.
Figure 4. A view of reconstructed model of the Pfyffer’s relief
(created using the software Skyline)
The complete digital data set of Pfyffer's relief was archived at
the Kulturgiiterschutz of Lucerne for the documentation of
cultural heritage. In case of the damage of the relief or its parts,
the precise digital data can be used for physical reconstruction
of the original.
In order to determine the accuracy of Pfyffer’s relief, the
reconstructed model must be compared with current map
information. Considering relief distortion in all three directions,
various methods of spatial geometric transformations have been
implemented and solutions are proposed in this section. In
particular, a comparison of the historical terrain model with the
current data represents a new problem in the historical
cartography: rather than rectifying and georeferencing a planar
old map, the 3D model has to be analysed. An important goal of
accuracy analysis is a good visual presentation of the work; the
methods and results must be easily understandable for project
partners - historians with less technical background. As
programming languages C and Matlab are used. The input data
and all the results are georeferenced, maintained and visualised
in ArcView GIS.
Two data sets are used for the comparison:
1. Historical data in the local coordinate system: DTM of
1 cm grid width, orthoimage of 0.5 mm footprint
(13T20 x 7780 pixel) and structured 3D vector data.
The data set covers the whole relief area (6.7 x 3.9 m 2 ).
2. Current data in the Swiss national coordinate system:
DTM of 25 m grid width, digital map 1:25000 with 2.5
m footprint and structured 2D vector data VECTOR25.
The data set covers an area of about 96 x 87 km 2 .
The accuracy analysis of Pfyffer’s relief is based on a number
of identical points selected according to principles of historical
cartography. The procedures described in the following include
definition and transformation of identical points, transformation
of image, height and vector data, visualization of the results,
analysis of lake contours accuracy and the DTM comparison.
4.1 Definition and transformation of identical points
Identical points are objects in Pfyffer’s relief, which in reality
have existed for centuries and which can reliably be found in
both data sets: churches, crossings, mountain peaks, bridges
etc. For each point, 3D coordinates x, y, z of historical data in
the local system and X, Y, Z of current data in the national
coordinate system are stored. Additionally, identical points are
assigned to one of three categories according to their estimated
reliability. In co-operation with historians, overall 221 well
distributed identical points were defined (Figure 5).
Figure 5. A chapel on a lake island as a reliable identical point,
left in Pfyffer’s relief, right in the 1:25'000 map
To determine the absolute accuracy of Pfyffer’s relief, identical
points have been transformed using a spatial transformation
with 9 degrees of freedom - 3 shifts, 3 rotations and 3 scales,
which proved to be particularly suitable for old relief models
(see Niederoest, 2002a):
m y
, m z)
where x, y, z = coordinates in historical data set
X, Y, Z = coordinates in current data set
d x , d y , d z = shifts in three coordinate directions
m x , m y , m z = scales in three coordinate directions
a, P, y= rotation angles
R = rotation matrix