International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
  
geometrically, should be used, so that the displacements could 
be corrected straightforward. However, after the correction of 
the objects displacements there remain empty areas without any 
image information. These so called gaps necessitate filling up 
with corresponding data from other imagery. Then the result of 
the described process is a true orthoimage, which contains all 
objects in their correct position. The procedure and the 
geometric conditions are well understood and published many 
times in the literature, e.g. Mayr (2002), Braun (2003). 
But to put this common knowledge into practice is very 
difficult, due to the fact that the data acquisition for detailed 
modelling of the objects above the topographic surface by 
photogrammetric methods is a very complex, sensitive and 
time-consuming task. Even new techniques like laser scanning 
methods could not solve the problem. However, a digital ortho- 
image is only as accurate as the surface model provided. 
Through the development of optoelectronic pushbroom scan- 
ners a totally new approach for orthoimage generation became 
possible. It is based on the special geometry of the new camera 
systems, which should be shortly demonstrated. 
3. PUSHBROOM SCANNERS 
AND THE NEW APPROACH 
In principle the pushbroom scanners follow the three-line 
concept developed by Hofmann (1982). In general, there are 
three (or more) CCD-lines which acquire image data of the 
terrain surface continuously during the flight. At least one of 
the lines is nadir-looking and thus provides image data in 
parallel projection along the flight direction and in central 
perspectives across (fig. 1). This means that — ideal flight con- 
ditions assumed — the captured objects show no displacements 
and are provided in their correct position in flight direction. It is 
of particular importance that these results are independent from 
the object heights. These geometrical properties have never 
been considered in the traditional approaches for orthoimage 
generation. 
  
Figure 1. Data acquisition of a 3 line pushbroom scanner in 
principle 
Now, the new approach makes use of this specific image 
geometry for producing true orthoimages. For this purpose, the 
surface must be imaged twice in two flight lines perpendicular 
694 
to each other. Thus, information about the correct location of 
any point is given in two directions. This fact is utilized by 
combining both data sets in order to derive data in parallel 
projection, i.e. true orthophotos. Figure 1 shows the digital 
airborne data acquisition by a 3 line scanner schematically. It is 
sketched, that the overflown surface is imaged by three sensor 
lines a, b and c, located in the focal plane of the camera lens. 
Under regular flight conditions of the airplane the sensor line b 
observes a differentially narrow line g of the terrain surface. By 
a uniform forward motion of the acquisition system along the 
flight line F and a constant appropriate recording rate, a strip of 
image data will be recorded which presents the surface in 
parallel projection in flight line direction. Due to the fact that 
object displacements only occur along the sensor line, an 
object, e.g. a house H above the reference plane in figure 1, is 
leaned outwards in line direction. Thus, it exists a true ground 
coordinate value in the flight direction, and this is independent 
from the height of the object. This knowledge about the mapped 
points in flight direction wasn't used so far. As already 
mentioned, the new approach combines two strips in parallel 
projection in order to derive a pair of ground coordinates with 
correct values in both directions. 
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TO 
Figure 2. Schematic generation of the true orthoimage by 
means of the new approach 
Figure 2 illustrates the main principles of the new approach. 
The surface of the illustrated object, which perhaps represents 
the roof of a building H, is imaged at position H1 by flight F1. 
At the second level in figure 2 the same surface is imaged 
towards the other coordinate direction by flight F2. By use of 
the given coordinates, the image pixel or segment H can direct- 
ly be mapped into the matrix of the true orthophoto TO. 
It is evident that the main issue in this approach is to locate 
corresponding points in both data sets and to connect them to 
the correct attitude information. This could be achieved in two 
different ways. The first one is the application of matching 
techniques as they are well established in digital photogram- 
metry (see e.g Scholten et al. 1998, Wewel 1996). The second 
possibility for solving this task is making use of segmentation 
procedure in order to map corresponding image segments into 
the orthoimage. In this manner, the object displacements can be 
corrected with the knowledge of the proper location coor- 
dinates, and the final result will be a true orthoimage. 
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