Full text: New perspectives to save cultural heritage

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 

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