International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
During the course of this development, a number of interested
organizations participated in addition to the USGS. The
software development was a cooperative program between The
Ohio State University and the University of Calgary with a
grant from USGS. The Madison Test and Calibration Range
was developed and maintained by the Ohio Department of
Transportation / Aerial Engineering Office. Flight testing of
the digital cameras was accomplished by Topo Photo, Inc. of
Columbus, Ohio.
2.1 In situ Approach to Calibration
In a benchmark publication by Eisenhart (1962), found in the
Proceedings of the National Bureau Standards, a rationale for
the calibration of measurement systems is set forth. His work
provides clear guidance for designing and applying a calibra-
tion program for the airborne sensors associated with the aerial
mapping industry. To summarize Eisenhart’s concept of
calibration:
e Establish. first... the
Specifications”
e [Exercise the measurement system as specified and
compare results to a standard of higher accuraey until
sufficient information is available to achieve a “State
of Statistical Control"
"Measurement System
The system specifications describe all aspects of the system
including hardware, software, environment of the system and
operational procedures necessary to achieve the final
measurement. Procedures may include specified ranges within
which the system will operate. In the aerial case, for instance, a
range in altitude can be specified that, by its definition,
represents an important practical aspect of the concept.
By accepting the concept of measurement system calibration, it
is clear that certain aspects of camera calibration, as
traditionally practiced, need to be reconsidered. For the aerial
camera, independent of the added sensors, the primary
difference in measurement accuracy for application is due to
temperature differences between a laboratory and an in situ
method of data collection. This difference is most evident for
an open-port windowed aircraft, most typical of the aerial
industry. For the closed-port, the addition of the window adds
an additional optical component to the system not conveniently
treated during the laboratory approach to calibration.
Experimental data collected under applicational cireumstances
tend to support the need for adopting an in situ approach to the
calibration of the aerial camera.
For the aerial case, the choice of camera platform, as with
digital cameras, offers a wide range of possibilities with the
technically best being the most expensive. The choices.
discussed below, range from the minimum cost, single-engine,
open-port aircraft to the multi-engine, windowed-port aircraft.
2.1.1 Single-Engine Open-Port In the United States one
popular, relatively inexpensive, aircraft is the Cessna 207. The
aircraft represents an adequately stable platform for carrying
the camera, mount and supporting equipment. However, when
considering use of airborne GPS for survey control, one must
assume that a significant disturbance is generated in the volume
20
of air below the aircraft through which the camera must
function. The engine exhaust may be diverted, however, the
cooling air for the conventional reciprocating engine can cause
a rise in temperature from external ambient to cowl exit of
66°C. In addition, as with all open port systems, the influences
of temperature differences between cabin and external air will
have an influence on the metric characteristics of the camera.
2.1.2. Multi-Engine Open-Port Probably the most widely used
aircraft for colleting photography of photogrammetric quality is
a light twin, open port platform. With this aircraft, no
significant disturbance to the volume of air beneath the aircraft
is expected. However, the influence of the temperature
difference between cabin and outside air can be extreme. The
difference causes a change in relationship between the optical
and image collecting components of the aircraft. This change
usually is seen as a centering error that can be represented as a
corresponding change in the camera constant. Accordingly,
recording temperatures within the cabin and at the camera lens
may become a means for accounting for the open port
installation errors.
2.1.3. Multi-Engine Windowed-Port The ideal, but most
expensive aerial platform includes a windowed port. The
window consists of high quality glass as specified by military
standards and others. Clearly, in application, the window
becomes part of the optical system and must be included as part
of the photographic system during calibration. When operating
without cabin pressurization, the influences of temperature
differences are mitigated. When pressurized, the differences
between cabin and external pressure generates a stress/strain
relationship on the window, producing an image deformation,
that - requires additional mathematical modeling during the
calibration process (a component of the system specification).
3. USGS/OSU PROJECT
The USGS project is viewed as an initial step leading to a
means of camera system calibration on a national basis.
Software was developed for the image measurement process
and for the subsequent computation of interior orientation, the
primary components of the camera calibration.
Subsequently, a series of flights were conducted over the
Madison range to verify the systems approach to camera
calibration for both the digital and film-based cameras.
3.1 Software Development
The programming, accomplished in Visual C++ language,
resulted in two programs.
3.1.1 Image Measurement Program The first program,
termed “Image Measure” (IM) software, was designed for
measurement of image coordinates and production of files for
subsequent introduction to the calibration program. The
observation of target images is facilitated by computation of a
single photo resection after the first four targets have been
identified and manually measured. At that point, the program
indicates the residuals of the fit to control, and selects only
those targets that appear within the current photograph. This is
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