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ed. It is viewed
as one of several possible auxiliarv data which are used to
support block adjustment and thus the indirect method of
georeferencing. The direct method, in contrast, does not require
connectivitv information within a block of images to solve the
georeferencing problem and, thus. offers much greater flexibility.
It is especially intriguing to consider its use for close-range
imaging applications which use either digital frame cameras.
pushbroom scanners. or lasers as imaging components.
In the following. common features in the design and analysis of
mobile close-range imaging svstems will be discussed and
illustrated by examples. Many of these features are also
important for general multi-sensor systems. however, the
discussion of a morc narrow field simplifies the. presentation.
Svstem design and analysis comprises the following steps as a
minimum:
e Data acquisition
e. Svnchronization and georeferencing
e Integration and data fusion
e Quality control
e Data flow optimization and automation.
These processes will be brieflv discussed in the following
chapters. To illustrate the major steps. the development of the
VISAT system will be taken as an example. The design
objectives for this svstem were as follows (Schwarz et. al.
(1993b)) :
"A multi-sensor system is required that positions all
visible objects of interest for an urban GIS with an RMS
accuracy of 0.3 m while moving through a road corridor
with a maximum speed of 60 km/h and a maximum
distance to the desired objects of 30 m. Data acquisition
must be automatic and should contain real-time quality
control features. Data processing, except for quality
control, will be done in post mission and should have
separate modules for georeferencing, image data base
management, imaging, and quality assessment."
2. CONCEPT OF A MOBILE MULTI-SENSOR SYSTEM
USING CLOSE-RANGE IMAGING SENSORS
The conceptual layout and data flow of a multi-sensor svstem for
close-range mapping applications is shown in Figure (1). The
selection of sensors for such a system obviously depends on
svstem requirements, such as accuracy. reliability, operational
flexibility, and range of applications. The data acquisition
module has therefore to be designed keeping both the carrier
vehicle and the intended applications in mind. The data
acquisition module contains navigation sensors and imaging
sensors. Navigation sensors are used to solve the georeferencing
problem. Although a number of different systems are used in
general navigation, the rather stringent requirements in terms of
accuracy and environment make the integration of an inertial
navigation svstem (INS) with receivers of the Global Positioning
System (GPS) the core of any sensor combination for an accurate
mobile mapping svstem for short range applications. This
combination also offers considerable redundancy and makes the
use of additional sensors for this reliability purposes usually
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
unnecessary. However, the addition of an odometer, such as the
ABS, may be useful for operational reasons, as for instance
keeping a fixed distance between camera exposures.
Imaging sensors can be subdivided by the way they contribute to
the information about the object space. They may provide
descriptive information, as for instance grey scales, or geometric
information, as for instance direction or ranges from the camera
to the object. Table 2 summarizes the contribution of sensors
typically used in close-range mapping applications.
In close-range mapping, photogrammetric methods have been
increasing in importance, due to the use of CCD cameras. These
sensors have overcome two major disadvantages of film-based
photographic cameras: single-frame, slow-rate photography and
highly specialized processing equipment. Recent trends in CCD
technology are characterized by increased resolution, color image
acquisition and improved radiometric quality (anti-blooming,
reduced cross talk). Another important development which
supports thc usc of CCD cameras in photogrammetric
applications, is the advancement of fast analogue-to-digital
conversions (ADC). Frame grabbers integrated with high-speed
computer buses and processing hardware have become a
standard commodity. Compared to analog/analytical plotters
used in conventional photogrammetry, the use of state-of-the-art
computer image boards greatly simplifies measurements.
The selected sensor configuration requires a certain data
processing sequence. Part of the processing will have to be done
in real time, such as data compression for the imaging data and
initial quality control processing for the navigation data. Most of
the data, however, will immediately be stored away for post-
mission use. In post-mission, the data processing hierarchy is
determined bv the fact that all images have to be georeferenced
first before they can be used in the integration process. The first
step is therefore the georeferencing of all recorded images and
their storage in a multimedia data base. To determine 3-D
coordinates of objects visible in CCD camera images, the
following information is needed for a pair of cameras:
e Position of the camera perspective center at exposure time (
3 parameters per image).
e (Camera orientation at exposure time ( 3 parameters per
image).
e Interior geometry of the camera sensor.
e The lens distortion parameters
The first two set of parameters are known as exterior orientation
parameters, while the other two sets are known as interior
orientation parameters. The general problem in photogrammetry,
aerial and terrestrial, can be seen as the determination of the
camera's interior and exterior orientation parameters. The
exterior orientation parameters are determined by a combination
of GPS and INS, the interior orientation parameters by
calibration. This means that exterior orientation is tied to a rcal-
time measurement process and its parameters change quickly. In
contrast. interior orientation is obtained by using a static field
calibration procedure and can be considered as more or less
constant for a period of time. Thus, it can be done before or after
the mission and is of no concern in the data acquisition process.
