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International Archives of the Photogrammetry, Remote Sensing
and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
proper surface modelling. Nevertheless, the photographs
affected the overall geometric quality positively, thus improving
also the laser measurements. Points on the object, which are of
specific interest, of course, have an improvement in accurary of
a factor of 3 (see Tab.3, row *'Tie pts (statue)") where the mean
location accuracy of one point could be reduced from 3.6 mm to
].2 mm.
| Laser Sc. Adjustment Joint Adjustment
Units in [mm] dX dY dZ dX dY dz
Laser position 2.1 2.1 32 1.9 1.8 29
Camera position | n/a n/a n/a 24122, 2.8
Tie pts (statue) 1.8 2.2 23 0.7 0.7 0.7
Tie pts (periph.) 3.9 3.6 3 3.3 3.1 3.5
Tie pts (mean) 234 25] 3| Sr 15) 15
Tab.3: Comparison of adjustments: Laser scanner alone and
laser scanner + digital photos
4. CONCLUSION
The examples above demonstrate the usefulness of combining
laser measurements with photographic pictures for a series of
c
reasons. Firstly, the photographs bear important and very
detailed information about the radiometric characteristics of the
object. One must also not forget a further advantage: a
photograph captures the object at a very short moment so that in
most cases motion effects caused by moving objects or moving
sensor platforms can be neglected. As typical examples the
dynamic acquisition modes of the CityGrid scanner have been
presented. Secondly, the information content of surfaces with
intricate intensity or colour textures can be reconstructed from
high resolution images. Thirdly, photos support the interpret-
ation of the object under investigation as a human operator,
who possibly controls the compilation process interactively or
at least evaluates the quality of the results, is used to this sort of
images. Fourthly, even if photographs are not needed for
fulfilling the requested surveying task, the high spatial quality
of image information can even help to improve the quality of
the laser measurements if used in combination, as the example
in 3.2 could prove. On the other hand, the advantages of the
laser measurement principle are apparent too: Firstly, the laser
scanner delivers a dense point cloud of polar measurements
with an rather high distance accuracy almost independent of the
surface properties. The surface need not be textured. Secondly,
at one single instrument position the entire hemisphere (or even
more) can be surveyed. Thirdly, laser measurements can
penetrate vegetation to a certain extent so that the reflected
pulse becomes a complexly shaped time-dependent signal.
Many of current instruments register either first pulse and last
pulse or even the full wave form which bears enough
information to derive object structures behind vegetation such
as tree or bushes even in urban environments. Fourthly, laser
scanning is an active technology and, therefore, does not
daylight or optimum illumination conditions. Laser does not
care of heavy cast shadows or severe brightness contrasts. Of
course, photo cameras too could be equipped with flash light,
but the usable range is rather limited. Fifthly, the laser scanner
almost immediately delivers a 3D data set, no further
complicated and time consuming compilation process like
image matching needs to be taken into consideration.
Each of the instruments has its great advantage. Technically it is
absolutely no problem to combine both acquisition techniques.
Modern digital cameras are of high quality and are available at
very low costs. If a laser scanning equipment is owned by
953
company, upgrading with a digital camera is highly
recommended. Ideally the camera should be integrated in the
laser scanner but, as the example above showed, cameras may
also be used independently. In any case, the reconstruction
process will benefit as far as the completeness, the reliability
and the accuracy, briefly, the overall quality is concerned.
Nevertheless, we are still at the beginning of the new
technology as far as the instrument developments and data
compilation is concerned. A further step forward will be the full
integration of photos and laser scans in the object
reconstruction process where also a higher degree of
automation can be expected — A great challenge for researchers
in photogrammetry, computer vision and surveying in general.
5. REFERENCES
Briese Ch., Pfeifer N., Haring A., 2003: Laserscanning and
photogrammetry for the modelling of the statue of Marc Anton.
International Archive of Photogrammetry and Remote Sensing,
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Cyrax, 2004: Leica Geosystems — Cyrax product information, at
http://www. cyra.com/products/HDS 3000 .specs.html (Link
checked, April 2004)
Forkert G., 2004: Der Einsatz von Laserscanning und Photo-
grammetrie zur Dokumentation des urbanen StraBenraumes.
Presented Paper, CORP 2004 & Geo.multimedia.04. Vienna.
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2, pp.613-619. Online proceedings at www.corp.at.
Haring A., 2003: Modellierung terrestrischer Laserscanner-
Daten. Diploma Thesis, I.P.F., Vienna Univ. of Technology.
Karner K., Klaus A., Bauer J., Zach C., 2004: MetropoGIS: A
city modeling system. Presented Paper at CORP 2003 &
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Schrenk M., pp. 199-203. Online proceedings at WWW.COrp.at.
Minolta, 2004: Product information by Minolta Europe GmbH
at. http//www.minolta-3d.com/products/eng/vi900.pdf — (Link
checked, April 2004)
Orpheus/Orient 2004: Product information by LP.F. at
http://www ipf.luwien.ac.at/products/products. html (Link
checked, April 2004)
Riegl, 2004: Product information by
http://www.riegl.com/834628/64221657.pdf
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Ullrich A., Schwarz R., Kager H. (2003): Using Hybrid Multi-
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System. In Grün, Kahmen (Eds.): Optical 3D Measurement
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Riegl Austria, at
(Link checked.
ACKNOWLEDGEMENT
This work was to a certain extent supported by Riegl Laser
Measurement Systems who provided equipment and mainten-
ance for test measurements and pilot projects. Riegl also intends
to join the Christian Doppler Laboratory "Spatial Data from
Laser Scanning and Remote Sensing" which has been
established in 2003 at the Vienna University of Technology.
Part of this work has been carried out within this Laboratory,
where the company NoLimits is already partner and where the
research focuses in particular on hybrid adjustment and
reconstruction methods for the CityGrid Scanner.
