International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BS. Istanbul 2004
2. DIGITAL MEDIUM FORMAT FRAME CAMERAS
Focussing on the medium format segment only two different
groups have to be separated (Petrie, 2003):
The first one is based on traditional medium format analogue
camera housings where the film cassette is replaced by a digital
back only. Such medium format digital camera backs have been
developed from different manufacturers like Creo Inc. (Leaf
Valeo 22), Imacon (Ixpress), Kodak (DCS Pro Back),
MegaVision (S4), PhaseOne (H25, H20), SinarBron Imaging
(Sinarback 54, 44) to be used to modify the analogue cameras
offered i.e. by Contax, Hasselblad, Linhof, Mamiya or Rollei.
There matrix area typically varies from approx. 4000 x 4000
pixel to 5400 x 4100 pixel.
In contrary to this, the systems from the second group are
originally designed as digital cameras, where the
Redlake/Kodak MegaPlus camera series is one of the most
popular representatives. Quite often those MegaPlus cameras
are used as one sub-system of integrated airborne data
acquisition platforms consisting of laser scanners (LIDAR)
combined with imaging component and GPS/inertial sensors for
direct platform orientation. This approach is highly effective,
not only for classifying the returned laser pulses but even more
for rapid production of orthoimages based on the height data
obtained from laser scanning and the directly measured sensor
orientations. Such integrated airborne systems are operated
from many airborne companies. A good online link catalogue of
world-wide laser scanning system users is given by
www.airbornelasermapping.com (2004). Many of them are
using medium format cameras supplementary to the laser
height data.
Within the following figures two representatives of digital
medium format sensors are given. The Contax 645 camera
(Figure 1) is originally designed as classical medium format
analogue camera which can be extended by different digital
backs as mentioned above and therefore belongs to the first
group of medium format segment digital sensors. Different to
that the Kodak DCS Pro 14n camera (Figure 2) is based on a
standard analogue small format photography housing, where a
large format CMOS digital matrix array providing 13.9 million
total pixel is replacing the analogue small format film. From the
resulting sensor size of 24 x 36 mm (which exactly matches the
standard small format analogue film (no focal length
magnification effect)) these camera could also be assigned to
the small format group. Nonetheless, from the number of pixel
this camera is closer to the medium format sensor. Hence the
Kodak DCS Pro 14n is treated as representative of this class
segment here.
Figure 1: Contax 645
(O Kyocera 2004).
Figure 2: Kodak DCS Pro 14n
(© Kodak 2004).
To resume, the use of medium format digital cameras might be ¢
quite attractive alternative for certain applications which are
more focused in smaller projects with reduced spatial
extensions or lower resolution imagery utilizing higher flight
heights, mainly for photointerpretation and classification
purposes. Although the price for such medium format sensors is
higher compared to the small format systems up to 6 Megapixel
the number of images to cover a certain project area is
significantly less which simplifies the later processing. And
still, the financial budget is significantly lower compared to the
very costly large format sensors. From this point of pricing the
Kodak DCS Pro 14n is advantageous combining the classical
small format film housing (consumer product) with a high
resolution 13.8 Megapixel matrix in total which is almost in the
15 Megapixel medium format segment.
Some of those digital sensors are additionally combined with
GPS/inertial components for direct exterior orientation
measurements. Such combination with GPS/inertial components
results in a significant price increase especially when using the
very high-quality GPS/inertial products (i.e. Applanix POS/AV,
IGI AEROcontrol). Such systems are more expensive than the
camera body itself but the combination of both offers the
possibility for the very fast production of geo-referenced
products. It gives more flexibility, and the fast turn-around of
geo-referenced products is highly desirable especially for digital
imagery. Such integration is standard when using the digital
camera as sub-system for integrated airborne data acquisition
platforms, as mentioned above.
3. REQUIREMENTS FOR NON-DEDICATED
AIRBORNE SENSORS
3.1 Influence of sensor format
Using digital frame cameras for airborne data acquisition the
size of the matrix array in combination with the used optics
(focal length, lenses) is of major importance. The dimension of
the used matrix array taking into account the individual pixel
size is relevant for the achieved base-to-height ratio 4 = B/h, .
This value reflects the geometry during image recording and is
one main factor for the resulting point accuracy in object space.
From the well-known normal case equations the influence of
base-to-height ratio for object point quality is obvious. From the
following equation the influence on 3D object point accuracy
0,,0,,0, is given,
where o,.,0,,0, reflect the accuracy of horizontal parallaxe
ot
and image coordinate measurements in x and y, respectively
(Kraus, 1990). As it can be seen from the formulas, for high
performance object point determination a large base-to-height
ratio should be aspired. For analogue photogrammetric cameras
values of 1.08 (super-wide-angle, focal length 8.5cm) or 0.61
(wide-angle, focal length 15cm) could be achieved assuming
standard 60% forward overlap conditions. In principle, similar
values are feasible for digital sensors. Since many of the digital
sensors are using normal angle optics due to the virtual
magnification of focal length caused by smaller format matrix
