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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
  
Each of these 13 CCD sensors is the front end of a separate 
imaging module. It consists of the sensor, the sensor 
electronics, a high end analog/digital converter (ADC), a fast 
digital signal processor (DSP) and the IEEE 1394 data transfer 
unit. 
The raw image data is transferred via the IEEE 1394 interface 
to a separate storage module of the Storage and computing Unit 
of UltraCamD. 
Thus the camera offers a frame rate of more than 1 frame per 
second, exploiting the benefit of its parallel system architecture. 
The panchromatic image consists of 11500 pixels cross track 
and 7500 pixels long track. Color is simultaneously recorded at 
a frame size of 4k by 2.7 k pixels for red, green, blue and near 
infrared [Leberl 2003]. 
The Storage and Computing Unit (SCU) of UltraCamD is 
responsible for the raw image data storage. It offers an entire 
storage capacity of 2 * 780 GByte, i.e. space for 2692 images 
stored twice on two separate sets of HDDs. 
The system architecture shows again a highly parallel concept, 
each CCD unit of the camera head (Sensor Unit) has its own 
"private" storage unit, including a small scale computer and a 
set of two mirrored and sealed HHDs. 
In addition to the storage capacity the SCU offers calculating 
power for the postprocessing by exploiting its 15 processor 
units. 
3. FROM 60% TO 90 % FORWARD OVERLAP 
The traditional photogrammetric workflow is based on 60% 
forward overlaps. Stereo operations strictly employ. 2 
intersecting projection rays, ignoring the lack of sufficient 
redundancy. With the advent of digital sensors, the number of 
images is no longer a factor of the project costs, as long as 
processing is automated. This encourages thinking about the use 
of higher than 60% forward overlaps (Fig. 2). 
The benefit of such novel strategy is manifold. Most important 
is the fact, that each position on the ground is mapped 5 times at 
an overlap of 80 % and 10 times at an overlap of 90 % vs. the 
2.5 times appearance of a terrain position at 60% overlap. 
UltraCam is able to collect up to 2700 images per mission at a 
frame rate of up to one frame/second and therefore supports 
routine use of 80% forward overlaps in almost all 
circumstances. Smaller urban mapping projects might benefit 
from the use of even 90% forward overlaps. The aero- 
triangulation will become more robust since mismatches of 
tiepoints will disappear. DEMs will also be without mismatches 
and all terrain segments will have coverage. An integrated 
system with = geo-positioning tools and  multi-ray 
photogrammetric processing will result in a DEM-robot and 
will also produce true orthophotos robotically. 
The high overlap can be produced by increasing the frame rate 
during the photo mission. Taking into consideration an aircraft 
speed of 75 m/sec and a frame rate of one frame/second a 
minimum base length of 75 m can be obtained. In the case of a 
7500 pixel image dimension along track the 60 % overlap 
causes a base of 3000 pixels, thus a minimum GSD of 25 mm 
(50 mm at 80% overlap and 100 mm at 90 % overlap) can be 
achieved. 
Additional flight lines to improve the side overlap from 30 % to 
60 % can be considered for dense built up areas, but will cause 
additional costs. In dense built up areas such flight concept is 
then able to avoid nearly any occlusion and therefore offer a 
optimal dataset for true ortho production. 
  
  
  
Fig. 2: Sample of a flight line with 60% (top), 75 % (center) 
and 87 % forward overlap (bottom). This corresponds to a 
relative base length of 40%, 25% and 13% of the image 
dimension. The number of frames doubles between each of 
these examples. 
4. DIGITAL VS. FILM SENSING 
The digital sensors have advantages over film due to the 
absence of grain noise, due to the dynamic range at 12 bits per 
pixel as opposed to «8 bits in film, and due to an inherent 
geometric stability because of the absence of moving elements 
and of photo laboratory processes. 
  
Figure 3: Comparing aerial film and digital image. Left is 
scanned film at a pixel size of 15 um (GSD of 15 cm); at right 
is the digital image with a GSD=17cm (from Leberl & Gruber, 
2003).