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The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
GSD*n 
pa (1—£-) 
100 
(1) 
The minimum exposure interval of the digital medium format 
cameras is somewhere between 2 - 3 s. 
Figure 2 provides an overview which minimum GSD for a 
photogrammetric aerial survey with an endlap of 60 % applies 
at what speed over ground. 
30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 [m/s] 
58 68 78 87 97 107 117 126 136 146 156 165 175 185 194 [kn/h] 
Speed over Ground 
Figure 2: Exposure interval of medium format cameras related to GSD 
and ground speed 
3.3 Image motion 
image noise levels. The aperture is also a limiting factor, 
because a wide aperture may cause or enhance vignetting and 
optical aberrations. To sum up: the image motion limits the 
GSD of medium format cameras and the short exposure interval 
necessary limits the flying operation times under poor lightning 
conditions when compared to large format cameras. 
4. GEOMETRIC AND RADIOMETRC CALIBRATION 
The calibration of digital cameras includes geometric and 
radiometric issues. From the viewpoint of a photogrammetrist, 
geometric issues are the more important. For large format 
cameras detailed tests within the frame of OEEPE and 
EuroSDR were conducted, e.g. Cramer, 2007. The special 
geometric problems of multi head cameras are discussed quite 
extensively in the literature, e.g., Jakobsen, 2007. Research in 
the calibration of large format cameras systems is an ongoing 
process with the aim to develop a calibration procedure of the 
whole camera system and its subsystems, including GPS/INS, 
radiometric and geometric issues and the whole 
photogrammetric processing chain, EuroDAC 2 (2008). 
4.1 Geometric calibration 
Due to the compactness and the low weight of medium format 
cameras, laboratory calibration of the interior is relatively easy 
to obtain. As mentioned in section 3.1.1 the interior orientation 
of medium format cameras may change under airborne 
conditions. Therefore the geometric calibration of the medium 
format camera system should be done in four different levels: 
Airborne images are acquired from moving platforms such as 
aircrafts or helicopters. The movement of the sensors during the 
exposure influences the quality and the sharpness of the 
acquired imagery. For analogue airborne cameras this image 
motion is taken care by forward motion compensation (FMC). 
For large frame digital airborne sensors the translation effect of 
the FMC is solved digitally by moving the charges on the 
matrix area itself (time delayed integration, TDI). Additional 
rotational movements are compensated from the stabilised 
mount. For medium format sensors an active mount is typically 
not available - exemptions are the DSS and cameras which are 
used as a sub-system e.g. in combination with laser scanners 
mounted on a common platform which is then stabilised. Also 
TDI is not available for the medium format digital sensors due 
to the Bayer pattern of the CCD-chip. The image motion u is 
related to the aircraft velocity over ground v g , the exposure time 
t e , the focal length c, the flying height above ground h g and the 
size of the pixel s p see formula 2. 
2 
GSD 
(2) 
where only 50% of the theoretical image motion u th is valid in 
the images. For digital imagery the smear due to image motion 
should not exceed 0.5 pixel. Since aircraft velocity and GSD are 
typically given by default for a certain project, exposure time is 
the only variable to minimise effects of image motion, if 
suitable light conditions are available. Exposure time on the 
other hand is coupled with lens aperture and the sensitivity of 
the digital sensor given by the ISO value. However a higher 
sensitivity (ISO number) is always associated with higher 
1. Laboratory calibration with a 2-D or 3-D test field. 
2. In flight calibration over a calibration range. 
3. Simultaneous in flight calibration on the job to adjust for 
project specific circumstances 
4. Long term camera stability analysis to determine the 
necessary calibration intervals. 
4.2 Radiometric accuracy and calibration 
An increasingly important task is the “radiometric accuracy” 
and the radiometric calibration of aerial cameras. The first goal 
of such a radiometric calibration is to eliminate the influence of 
the optics and the sensor and make sure that the resulting 
images will have the same sensitivity throughout the image. 
The radiometric calibration is also split in two main parts. The 
first part of the radiometric camera calibration is done by the 
manufacturer to eliminate radiometric dysfunctions of the 
sensor such as: 
• Defect pixels 
• Dark Signal Non Uniformity (DSNU) 
• Individual sensitivity of each single CCD pixel 
• Vignetting (partly) 
• Influence of aperture (partly) 
However, the manufacturer radiometric calibration effort differs 
between medium format cameras. White balance calibration 
procedure or more general the Look-Up-Table (LUT) 
generation is the second step. For each project the user performs 
this type of calibration individually. After post processing the 
user can set the white balance for the project using some 
example images which cover the typical surface,. With suitable