In: Wagner W., Szekely, B. (eds.): ISPRS TC VII Symposium - 100 Years ISPRS, Vienna, Austria, July 5-7, 2010, IAPRS, Vol. XXXVIII, Part 7B
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3.2 Analysis
The results of the two calibration procedures indicate that the
parameters are close to being identical (Table 2). The charts in
Fig. 7 also show the same phenomena. However, in our
experiments, we found that the results start varying if the
camera is positioned too close to the targets. This observation
seems consistent with previously reported studies on close
range photogrammetric camera calibration (Brown, 1971).
However, more analysis needs to be done for anything
conclusive. Since the Box as a calibration target is meant for
small format short focal length cameras, the distance between
the targets and the cameras should be close enough so that
Australis software is able to recognize the targets. The size of
the targets, therefore, needs to be selected accordingly.
4. CONCLUSIONS
In this research, two methods of camera calibration that are used
at the USGS EROS at Sioux Falls, South Dakota, USA were
presented. The camera calibration lab is housed primarily to
calibrate medium format digital cameras, with a focal length
range between 20-120mm. The main calibration method uses
the principles of self calibration and bundle adjustment on
coded targets located on an aluminium cage. A second method
to perform calibration was presented. This method used a scaled
down version of the coded targets pasted on a small rigid box.
Both the methods involve taking images of the targets from
different camera locations and orientations. The solution to the
bundle adjustment problem is obtained using the software
Australis. It was shown that the solutions camera calibration
parameters obtained from both the methods are close to each
other. The same time the approach using the box yields
promising results and can be used for verification of the
calibration parameters. Further research on the box by adding
more targets may yield results closer to the results obtained
from the cage. There has been an increasing interest in
calibrating longer focal length cameras (> 150mm) using self
calibration methods. The problem becomes non trivial given the
limitations of space. Further research is being conducted at the
USGS on expanding the range of cameras, with regards to the
focal length, that can be calibrated in the lab.
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DISCLAIMER
Any use of trade, product, or firm names is for descriptive
purposes only and does not imply endorsement by the U.S.
Government.
ACKNOWLEDGEMENTS
Sam Johnson and Alonso Holmes performed most of the camera
calibration experiments