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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXX V, Part B6. Istanbul 2004
necessary. Due to strong correlations between some of the
calibration parameters and exterior orientation elements, the
block layout should consist of two flight lines forming a cross,
each line flown twice in bi-directional flight directions. In
principle, such pattern is sufficient to estimate all parameters
(even without additional ground control) except of the focal
length distance. To estimate this parameter, the knowledge of a
scaling factor is necessary, which can be obtained from
introduction of ground control. Alternatively the same
calibration block could be flown within a different flying height
resulting in two different image scales. Since both blocks are
connected via tie points, such block layout not only allows for
calibration without any ground control but also has advantages
in terms of stronger block geometry, which results in very
reliable estimations of calibration parameters. Hence this double
cross block layout is the recommended pattern for calibration
flights.
Practical tests have shown, that based on this self-calibration
procedure an accuracy of 2.5-2.91um is obtained for all ADS40
systems, which is the accuracy potential to be expected from the
automatic tie point matching quality. Since the additional 6"
order polynomials are non fully integrated in the bundle
adjustment (status 2003) the final self-calibration parameters are
obtained from 4-6 iteration steps. It is worth to mention, that
starting from the values obtained from lab calibration, only one
single iteration step can be saved. From this background first
trends are visible to obtain ADS40 camera calibration
parameters from self-calibration exclusively. Potentially,
ADS40 lab calibration will totally set away in future.
The calibration results are documented in a 5 pages long
calibration certificate. Within this document the tested
individual system components are given and the layout of the
calibration flight with tie points is depicted. The calibrated
misalignment angles are given, the results of geometrical
calibration (i.e. calibrated x/y coordinates of all pixels of all
sensor lines) are not mentioned explicitly — they are attached
separately in a digital file, which belongs to the certificate.
S. SUMMARY
Although this report on the today's status of digital airborne
camera calibration is only on its first preliminary stage, these
comments will be the base of a more detailed report, which will
be published within the next months as result of the first phase
of the the EuroSDR project on “Digital Camera Calibration”.
This Phase 1 final report is open to all persons interested in the
different methods of digital camera calibration.
Although only the geometrical calibration of three airborne
systems was described in more detail in this paper, some general
trends are clearly visible:
- System driven calibration approaches are gaining in
importance due to the complexity of digital sensor systems
consisting of several sub-components.
— A decrease of importance of lab calibration seems to be
visible, whereas the importance of in situ calibration (i.e.
self-calibration based on distinct calibration flights) is
definitely increasing.
- The acceptance of such combined lab and in situ
calibration might be low from today's point of view and
has to be increased. This fact is caused from some
knowledge deficits on the users side, especially when
focussing on the full system calibration based on in situ
calibration techniques, which are not as common in the
traditional airborne photogrammetry field. With their
209
increased future use such methods will be accepted as
powerful and efficient tool for overall systems calibration.
All these aspects will be covered and discussed in more detail in
the ongoing project. Hence anyone being interested in these
topics is cordially invited to actively participate in the EuroSDR
network. All relevant information are available from the project
WWW side http:/www.ifp.uni-stuttgart.de/eurosdr/ .
ACKNOWLEDGMENTS
The author would like to thank all experts providing material
for compiling this paper. The continuous support of the
EuroSDR core group members is gratefully acknowledged,
where the advice of Prof. Eberhard Gülch should be highlighted
especially. Finally, deepest thanks is expressed to Prof. Gordon
Petrie for his competent and detailed contributions to system
and calibration topics.
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