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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXX V, Part B7. Istanbul 2004
building facades have to be plotted, where the projection plane
for the orthophoto can be positioned approximately parallel to
the main plane of the facade. An additional layer of the
orthophoto contains the z-distance of the facade points above
the reference plane, i.e. the distance of each surface point from
the projection plane of the orthophoto. The ZOP is a 2!4D data
set and can, in general, provide only a simplified representation
of a facade, as overhangs with respect to the ZOP projection
plane cannot be modelled. It has to be explicitly mentioned that
a ZOP is a digital product rather than a graphical representation
like a conventional orthophoto. It is actually a digital surface
model of the facade draped with the image of the facade.
Therefore, the ZOP can be used, in a similar way with similar
tools as for DTMs, for creating virtual perspective views,
calculating profiles across the facade in any direction, showing
visualisation via a 3D viewer, and last but not least, generating
true orthophotos. The latter term is justified as very small
details can be modelled due to the high resolution of the laser
scanner. If the pictures are taken from the scanning direction the
problem of multiple mapping, which could only be avoided by
strict visibility analysis, does not appear, although occluded
areas are likely to exist. Fig.3 shows a part of a true orthophoto
mosaic derived from a ZOP and a series of original oblique
images taken from a mobile city scanner. The ZOP may serve as
a valuable product for archaeologist, architects, etc., who do not
have photogrammetric expertise nor an appropriate workstation.
They can use a standard CAD program like AUTOCAD for
visualisation, measurement and even simple reconstruction.
Fig.3: True orthophoto and examples of original images
For more sophisticated object reconstruction the laser point
clouds together with the images have to bé analysed. Due to the
huge amount of data to be processed within one project
automated methods are required. Many research activities have
been started worldwide. The above mention LS&RS conducts
several projects which make use of the combination of laser
Scanning and photography for detailled object reconstruction.
The focus is on the development of interactive semi-automatic
methods rather than fully automated procedures.
951
3. EXAMPLES
3.1 The CityGrid Scanner
The CityGrid Scanner (CGS) by NoLimits is a mobile universal
multisensor platform consisting of a 3D Scanner, several high
resolution digital cameras and a GPS receiver. Its main area of
application is the fast and efficient acquisition of building
facades along roads and the 3D city environment of local areas
of special interest. The GCS may be operated in basically four
modes:
- Dynamic Mode (DYMode)
— Dynamic Stop Mode (DSMode)
— Stop & Go Mode (SGMode)
— Street Scene Mode (SSMode)
While the main acquisition device in DYMode and DSMode are
the digital cameras, in SGMode and SSMode the laser scanner
comes into action. Fig.4 demonstrates the principle of the
different modes as ground sections of coverage plots.
5GÀode
WIN
Fig.4: CityGrid Acquisition Modes (Left: Camera/Scanner
Modes; Right: Pure Scanner Modes)
The dynamic modes are used for quick building acquisition
with the main intention to generate facade image mosaics. The
vehicle may constantly move along the street and photo shots of
the facade are triggered at certain distance intervals so that a
sufficient overlap of information is guaranteed. The orientation
of the pictures is based on a line matching process between
images, which can run semi-automatically. In order to keep
acquisition time as short as possible, laser scanning may be
reduced to horizontal profile scanning only, thus delivering a
longitudinal profile of the building block. The profiles serve as
aid for orientation as they may be treated as approximate
ground section of the building blocks. Satellite based
navigation systems are not suited for reliable use as especially
in old towns with their narrow streets the GPS signal cannot be
received most of the time.
The orientation approach of the images has been developed by
NoLimits in co-operation with the K-plus Research Centre
VRVIS (Vienna, Graz) and uses horizontal and vertical edges
and vanishing points. Great overlap of the individual images is
a prerequisite (see lower image of Fig.3) (Karner et al., 2003).
In order to stabilise the geometry of a pure photo block (in
particular when surveying long facades in DYMode) and to
provide reliable control information at corners of building
blocks, where one photo series ends and another commences
possibly in another direction, 3D laser scans are taken. The
DSMode needs stops of the vehicle now and then. If the 3D
structure of the entire facade mosaic is requested, the stops must
be positioned so that the laser clouds overlap, too. This mode is
SGMode and may be combined with the DSMode-specific
acquisition so that both a complete overlap photo block and an
overlaping laser cloud can obtained. This later mode has to be
used, for instance, if ZOPs are to be created (see Fig.3).
Eventually the SSMode is intended for the acquisition of a
complete street scene, where hemispherical laser scans are
