The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
For the new approach the elimination of the pitch influence is 
difficult, because the effect depends on the height of the objects. 
That means, the higher objects stand out from the reference 
plane, the more precise navigation for the aerial survey must be 
carried out. If the pitch angles of the scan lines are close to zero 
then the displacements perpendicular to the line will be smaller 
and the true ground coordinates become more precise. 
The roll angle shows relatively quick and sudden changes in the 
raw image data and in the diagram. However rolling affects the 
new approach rather less. Thus the image points are also shifted 
here, but along the scan line. The influence has its maximum on 
the margin of the image strip, and is also depending on the 
height of the objects. However, this does not cause problems for 
orthoimaging, because the final ground coordinates are taken 
only perpendicular to the scan line (in flight direction). 
The yaw angle refers to the north direction, thus it is different in 
the absolute values for both flight strips (see Figure 4 below). In 
general the yaw angle effects an azimuthal rotation of the 
original scan line and leads to deviations in x- and y-direction. 
But the x-direction (perpendicular to the scan line) is more 
important, because in this direction the objects are shown in 
parallel projection. The differential changes of the yaw angle 
from scan line to scan line cause that the flight direction does 
not agree any more with the x-direction and that the x- 
directions of the original scan lines change in the corrected 
nadir image constantly, e.g. by a drift. 
However, a constant x-direction is essential for the new ortho 
imaging approach, because the correct ground coordinates are 
taken in this direction. If a constant x-direction is defined for 
one image strip, e.g. by an average yaw angle, angle differences 
appear in every line between constant and real x-direction. The 
angle differences lead to shiftings perpendicular to the scan line 
with a maximum at the margin of line. The shifting becomes 
too large and one gets incorrect ground coordinates, by 
processing the complete image strip with one average yaw 
angle. 
To solve this problem a subdivision of the overlapping image 
range into smaller segments with other x-directions seems 
feasible to hold the influence by the deviation of the individual 
x-direction low. As an example, the images in Figure 3 have an 
extent of 1000 pixels and a maximum angle difference of 0.2 
degrees. Thus the difference yields incorrect ground coordinates 
by 3 pixels at the end of the line. 
The differences between the yaw angles of both flight strips 
contain also possible effects of deviations from the ortho 
gonality between the flight lines. Also in this case the dif 
ferences are not constant from line to line. The deviations have 
its maximum on the strip margin and the effects are along the 
flight line. For the image segment in Figure 3 a constant dif 
ference was defined, thus the general part of the deviations can 
be attached to the incorrect ground coordinates, by a rotation. 
For the data investigated an average deviation from the ortho 
gonality was determined as approximately 1.7 degree. This 
leads to a correction of approximately 17 pixels across to the 
line. 
4.3 Occluded Areas 
A general problem of true othoimages are occluded areas 
behind buildings, bridges, etc. Rather sophisticated techniques 
must be applied to fill the resulting gaps in the images. The 
situation will be improved due to the mixed projection of 
pushbroom scanners, where relief displacements occur only in 
one direction. Theoretical studies show that through this effect 
occluded areas are significantly smaller in pushbroom datasets 
than in images acquired in central projection. That means, the 
necessary computations for the correction of occlusions and 
filling gaps in the true orthoimage will be reduced. 
5. DEFINITION OF CORRESPONDING POINTS AND 
TRUE ORTHOIMAGE GENERATION 
For general investigations about the realization of image 
matching with real image data the following simplifications 
were assumed: 
• the data acquisition of the data occurred in nadir direction, 
• the azimuthal orientation (yaw angle) of every scanned 
line corresponds to the middle flight direction. 
Furthermore following preparations were carried out: 
• geometrical and radiometrical corrected image data were 
used 
• the averaged flight directions were fitted to the coordinate 
axis’s so that the flight tracks were oriented in parallel 
with the respective axes of coordinates of the image coor 
dinates system. 
These preparations and simplifications enable a direct work in 
the scan lines and the determination of the correct coordinates 
from the image contents. 
For an exhaustive allocation of both images a correspondence 
analysis must be carried out for every pixel of an image. For 
detection of corresponding points in the two data sets the 
proven process of the normalized cross correlation was applied. 
For every pixel it delivers a statistical value, which shows how 
well the grey scale value variation of the searched pixel and its 
environment agree with the respective pixel of the other flight 
strip. The value range reaches from -1 to +1, and +1 is 100% of 
correspondence, 0 is no correspondence and -1 indicates an 
inverted correspondence. For the investigation only correlation 
values were used by at least 0.6 for the allocation of an image 
point. 
For the sizes of the template matrices, which contain the grey 
scale values of the examined pixel and its neighbors, values 
between 3x3 and 9x9 were selected. The correspondence 
analysis was carried out in both directions. Pixels from the first 
image was searched in the second one and vice versa. The grey 
scale values of the found points have been averaged. Because of 
the similarity of a many pixels often ambiguities led to wrong 
results. In order to minimize this effect the range to be 
examined (search matrix) must be limited. 
In the direction of central projection, objects which will stand 
out from the reference plane became displaced. In Figure 5 the 
displacement of some points is sketched schematically for both 
flight directions. The displacement of a point increases with the 
distance from the flight line. 
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