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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
Table | shows a comparison of several sensor characteristics. It
clearly demonstrates the partly low correlation between both
instrument properties what leads to the conclusion that both
echniques may complement each other in an ideal way.
Laser Scanner Photo Cameras
| Spatial Resolution High Very High
| Spatial Coverage Very Good Good
! Intensity/Colour Limited Very Good
Illumination Active Passive (& Active)
3D Pt Density High Depend. on Texture
| Depth Accuracy High High
Acq. Procedure Dynamical Moment Shot
( 3D Reconstr. Effort Medium High
Texture Reconstr. | No or Very Limited Very Good
Instrument Costs High Low
Table 1: Comparison of Sensors
While accurate and efficient photogrammetric reconstruction
procedures are state-of-the-art (many commercially available
software packages exist) and may be automated to a high extent,
processing and compilation of laser scans still need further
development. The hardware is still under development too and
significant improvements may be expected for the future,
although a high degree of precision, reliability, and practi-
cability has been reached. A wide range of laser scanner models
are available and several companies turned out to have gained a
significant position on the world market. One of those
companies is Riegl Laser Measurment Systems GmbH, Austria
(see ww w.riegl.com). Close co-operation exists with LP.F. and
in near future also with the Christian Doppler Research
Laboratory for "Spatial Data from Laser Scanning and Remote
Sensing" (LS&RS) which was founded in 2003 and is integ-
rated in LP.F.'s activities. The Austrian company No Limits IT
GmbH, a subsidiary company of Geodata Austria, (see
www.citygrid.at) is also partner of the LS&RS. This company
has developed a “CityGRID Scanner” (Forkert, 2004)
consisting of a Riegl Laser Scanner and a digital camera. The
device is mounted on a motorised vehicle which moves through
the road of a city and acquires building facades in three
dimensions together with the building texture. A highly auto-
mated reconstruction procedure for efficient scanning is
currently under development which should allow the generation
of a detailed virtual urban environment in a fast and affordable
way.
2. ORIENTATION AND RECONSTRUCTION
In order to achieve the optimum results in terrestrial laser
scanning, the utilisation of both range imaging and photo-
grammetric imaging is the preferred approach. Consequently,
also the reconstruction process has to concentrate on the
concurrent usage of both imaging products, by fusing the data.
The fusion can be applied during two fields of compilation:
*Orientation of sensors
* Object reconstruction
The first work is responsible for establishing a uniform and
geometrically consistent block of measurements from both laser
Scanner and the photographic images. Basically two set-ups are
possible. One set-up uses totally independent positioning of the
laser scanning device and the photogrammetric camera, the
other uses a combined, stable and calibrated arrangement of
laser scanner and camera. The geometric relation between
949
camera and laser scanning device is given by the so-called
mounting calibration, which has optimally to be determined in
advance and serves as a sort of interior orientation of the
acquisition system.
While the combination of both acquisition techniques offers a
series of advantages for the orientation of the sensing system, as
we shall see later, data fusion may also be used for the object
reconstruction procedure where range and intensity data
complement each other. Matching of individual scans and/or of
individual photographic images can be supported by 3D object
data as well as by 2D radiometric data. Thus, the geometric
accuracy, the reliability and the completeness of the recon-
structed object can be improved. In particular objects with
complex shape and/or texture benefit from the combination of
both acquisition techniques.
2.1 Orientation of the sensor set-up
2.1.1 General Remarks
In order to describe the geometric properties of the sensors and
in order to define a homogeneous coordinate system for the
object reconstruction, the different sensor observations must be
transformed into one global object co-ordinate system. À hybrid
adjustment approach provides the ideal means for determining
both the internal and the external sensor geometry. The Institute
of Photogrammetry and Remote Sensing of the Vienna
University of Technology has been working on problems of
bundle block adjustment techniques for years. The software
packages ORIENT and ORPHEUS are the result of this
research activities (Orpheus/Orient, 2004). This universal
approach realized in ORIENT can easily be adapted to various
acquisition techniques, even to the simultaneous orientation of
different techniques as in the case of laser scanning and
photographic imaging (Ullrich, et al., 2003).
For block adjustment tie points have to be measured which not
only tie the individual images and laser scans to each other but
also link the range measurements to the photographs. Control
points or control information, i.e. points with known position or
other known geometric properties with respect to a higher level
co-ordinate reference system, are necessary only to define the
orientation of the reference system. The scale can exactly be
determined by the range measurements. The translation. and
rotation parameters may, for example, be defined by one control
point and additional so-called fictitious observations, such as
horizontal plane, vertical plane, lines with known spatial
orientation, etc.
2.1.2 Solution in Riegl Systems
Riegl Laser Measurement Systems GmbH has recently started
to offer, as an option, their 3D laser sensors together with a
firmly mounted high-resolution digital camera. Due to the well-
known camera calibration and camera orientation with respect
to the scanning device the entire equipment can be treated as a
single hybrid acquisition system. The measurements are stored
on-line on a mobile computer (connected to the scanner through
a high speed data link) in an indigenous structured format
which contains all relevant exterior and interior sensor
parameters thus making separate alignment procedure
unnecessary. The data format is defined in XML and is open to
everyone thus enabling data export and import between various
utilities and any-post-processing software.
The software package RiScan Pro organises data acquisition,
calibration, visualisation and archiving, and prepares data for
