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New perspectives to save cultural heritage
Altan, M. Orhan

CIPA 2003 XIX th International Symposium, 30 September - 04 October, 2003, Antalya, Turkey
network, the availability of imaging cameras which can work in
combination with the scanner, weight and ease of transpor
tation, power supply (battery operation), availability and quality
of software.
Besides, the quality of the user support and the guarantee
conditions are not the same for all producers. These should be
checked carefully in addition to the technical specifications
before a decision is made to favor one product or another.
3.1 General remarks
When the decision was made to start a research program with
the aim to compare the accuracy and performance of different
types of laser scanners, new testing installations had to be
developed. In order to reduce measuring time and expenses, a
set of targets was designed using standard materials, and all
experiments were installed in two buildings of FH Mainz,
University of Applied Sciences. Most experiments can be
repeated at any other location provided the same type of targets
and surface paints are used.
Since single points of scans cannot be analyzed and compared,
ball type targets (white spheres with a diameter of 76.2 mm on a
magnetic ground plate as produced by MENSI) are used for
most distance determinations, and plane boards for experiments
concerning range noise and investigations concerning the be
havior of surfaces with different reflectivities. Some additional
special objects, described below, were constructed for further
It should be noted that these arrangements do not allow to find
the mechanical, optical or electronic sources of errors in the
instruments; instead they show the effects of such an error on a
certain measurement under practical measurement conditions.
When, for example, a distance between two spheres which are
at the same distance from the scanner, is derived after their
center points have been modeled from the point clouds, this will
give a general indication of the angular accuracy of the scanner
but does not really tell everything about the accuracy of the
angular position of a single point. This is basically the same
approach recommended by VDI/VDE in their guideline (VDI/
VDE, 2002). Since the same procedures and targets were used
for all instruments examined, this provides a reliable method to
compare the performance of these instruments under practical
application conditions.
3.2 Angular accuracy
Errors in the angles between two rays can be detected when the
distance between two spheres located at equal distances from
the scanner is determined. Modeling the spheres will result in a
low pass filtering. Therefore the results will not allow detecting
small arbitrary angular variations.
Fig. 1; Box for positioning spheres at defined locations on steps
A first test installation uses white spheres in a box that can be
positioned at well defined points on a stone stairway at the end
of a 60 m corridor. The box (fig. 1) allows repositioning the
spheres within some tenths of millimeters with respect to the
stone steps when the tips of six bolts protruding from the
bottom and the sides are brought in contact with the stone faces
of the steps. Thus, the precise position, acquired with geodetic
methods can be re-established any time. The targets are used on
either side of six steps at a distance of about 1 m (fig. 2). This
allows the calculation of six independent short distances in
horizontal and six in vertical direction.
Fig. 3: Steel plate for positioning sphere at a wall
In a different room, four spheres are installed at a vertical wall
at the corners of a rectangle 3.5 m high and 5 m wide. Since
special steel plates with two defined mechanical contacts for the
magnetic ground plates of the spheres are used (fig. 3), the
spheres can be re-positioned precisely to the original position
which was determined by geodetic methods. This arrangement
is scanned from a distance of up to 15 m from three observation
points as indicated in figure 4. This again yields six independent
distances in horizontal and six in vertical direction which can be
compared to their calibrated values.
Fig. 4: Location of spheres at a wall and observation stations