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New perspectives to save cultural heritage
Altan, M. Orhan

W. Boehler, M. Bordas Vicent, A. Marbs
i3mainz, Institute for Spatial Information and Surveying Technology, FH Mainz, Holzstrasse 36, 55116 Mainz, Germany,
Working Group 6
KEY WORDS: Laser Scanning, 3D Scanning, Close Range, Instruments, Accuracy, Calibration, Heritage Conservation.
Questions concerning the quality and accuracy of the recorded 3D points of laser scanners receive little attention. In a research
project, i3mainz has installed a number of different test targets that allow an investigation in the quality of points recorded by laser
scanners and the geometric models derived from the point clouds. The standardized tests also allow a comparison between
instruments of many different manufacturers for the first time. Seven instruments have been tested, more tests are already scheduled
for the near future.
Surveying results must meet certain specifications in order to
provide the necessary accuracy standards for a certain
application. On the other hand, if instruments and methods are
used which yield an accuracy far above the needed standard,
this will result in unnecessary cost and expenditure. Therefore,
any geometric surveying task comprises not only the derivation
of the relative positions of points and objects but also an
estimation of the accuracy of the results. Least squares
adjustment based on overdetermination usually yields a reliable
information concerning the accuracy of the results as well as the
accuracy of the observations. If the number of observations is
not sufficient for an adjustment, one may estimate the accuracy
of the results by propagating the errors of the observation
instruments to the results. In this case, the accuracy of the
measurement device has to be known.
In the case of laser scanners, a large number of 3D coordinates
on an object’s surface is measured in a very short time. While it
is possible to record the same object several times from
different observation points, it is impossible to record the very
same points in these repeated surveys. Therefore, deviations can
only be noticed after objects have been extracted from the point
clouds and modeled. If the geometric properties of the object
are known, however, the deviation of single points from the
object’s surface may be an indication for the accuracy. Using a
plane surface would be the simplest case, but cylinders or
spheres can also be considered.
2.1 General remarks
The accuracy specifications given by laser scanner producers in
their publications and pamphlets should always be doubted.
Experience shows that often these cannot be trusted and that the
accuracy of these instruments which are built in small series
varies from instrument to instrument and depends on the
individual calibration and the care that has been taken in
handling the instrument since.
Every point cloud produced by a laser scanner contains a
considerable number of points that show gross errors. If the
point cloud is delivered as a result of surveying, a quality
guarantee, as possible for other surveying instruments, methods,
and results cannot be given.
Many institutions have already published methods and results
concerning accuracy tests with laser scanners (e.g. Balzani et.
al. 2001, Johansson 2002, Kern 2003, Lichti et. al. 2000, 2002).
Based on this knowledge a comprehensive test program was
developed at i3mainz and as many different scanners as possible
are compared using the same installations.
2.2 Angular accuracy
The laser pulse is deflected by a small rotating device (mirror,
prism) and sent from there to the object. The second angle,
perpendicular to the first, may be changed using a mechanical
axis or another rotating optical device. The readings for these
angles are used for the computation of the 3D point coordinates.
Any deviations will result in errors perpendicular to the propa
gation path. Since the positions of single points are hard to be
verified, few investigations of this problem are known. Errors
can be detected when measuring horizontal and vertical
distances between objects (e.g. spheres) with the scanner and
comparing those to measurements derived from more accurate
surveying methods.
2.3 Range accuracy
In the case of ranging scanners, range is computed using the
time of flight of the laser pulse from the scanner to the object
and return. Ranging scanners for distances up to 100 m show
about the same range accuracy for any range. Triangulation
scanners solve the range determination in a triangle formed by
the instrument’s laser signal deflector, the reflection point on
the object’s surface and the projection center of a camera,
mounted at a certain distance from the deflector. The camera is
used to determine the direction of the returning signal. In
contrast to the ranging scanners, the accuracy of ranges
acquired with triangulation scanners diminish with the square