375 
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
oriented perpendicular to each other. The effects of de 
viations from these requirements are discussed in section 4. 
2. For the identification of corresponding image points 
effective matching techniques must be applied. This is 
discussed in section 5. 
For both aspects practical experiments have been carried out. 
The test data have been provided by the Institute for Planetary 
Research, German Aerospace Center (DLR). They have been 
acquired in an aerial survey over an urban area with the HRSC- 
AX in combination with high-precision DGPS and INS. This 
camera system, developed for aircraft operation, has a scan line 
of 12,000 pixels (Scholten et al., 2002). The area was flown 
from south to north and from west to east (marked with arrows 
in Figure 3) in an altitude of approx. 3650 m above the ground. 
The ground sampling distance was approx. 15 cm. The data 
were already corrected radiometrically, and the orientation 
parameters were provided for each individual image line. 
♦ 
ilifiBi 
plane (Figure 3 lower images). Such a rectification process was 
first established by Wewel et al. (1998). 
In this example the WGS84 ellipsoid was selected as the 
reference plane and the conformal UTM-projection with a 
locally defined middle meridian was applied. Due to the local 
UTM-projection and the small area, the influence of meridian 
convergence can be ignored. 
The deviations from the ideal configuration are significant. It is 
evident from Figure 3, that the simple image geometry gets lost. 
This means that the flight direction must not coincide any more 
exactly with the direction, in which the objects are shown in 
vertical parallel projection. This has a direct effect on the new 
process for the generation of true orthoimages. The influences 
of the aircraft motions can be estimated with the sensor rotation 
angles roll, pitch and yaw. For the calculation the boresight 
angles between the INS and camera system were used (see 
Sandau, 2005). Figure 4 shows the calculated orientation angles 
as well as the variations in both flight strips, containing 13000 
image lines. 
flight 1 
Ï#*« 
flight 2 
N.. 
Figure 3. The pushbroom scanner data of a test area with the 
flight directions shown on top, in the left image from south to 
north, and in the right image from west to east. Below, the 
flight motion corrected image data. 
4. GEOMETRICAL INVESTIGATIONS 
4.1 First Geometrical Correction 
From the images in Figure 3 it is clear that some geometric 
corrections are necessary before further processing. Due to the 
significant distortions in the raw data through aircraft motion 
and due to the deviations in position of the individual scan lines, 
the identification of corresponding points and the combination 
of the resulting ground positions could not be effective. Thus, 
the geometric distortions by aircraft motion must be corrected 
in a first step. 
This correction can be achieved utilizing the internal and ex 
ternal orientation data of the camera, which is available for any 
scan line. The correction of the image data refer to a reference 
Figure 4. Sensor movements roll, pitch and yaw for both flight 
lines. Roll in the upper left, pitch in the upper right diagram. 
Yaw for flight 1 in the lower left, for flight 2 in the lower right. 
4.2 Effects of Orientation Angles 
It is obvious from Figure 4 that the orientation angles change 
from one image line to the next. In order to study the influences 
of these changes the effects are discussed individually. 
The pitch angle is of special interest. It indicates the deviation 
out of the nadir direction, which has a constant effect on the 
complete line. The vertical parallel projection is changed to an 
oblique parallel projection. Thus, objects above the reference 
plane are displaced in adjacent lines dependent from the height 
of the objects. This relief displacement also yields an incorrect 
ground coordinate of an image point. As an example for both 
image strips: if an object height of 50 m is assumed and the 
pitch angle is 0,4 degrees the displacement will be 2 pixels or 
0,3 m.