a Digital Terrain Model (DTM). The geometric correction 
makes use of a ray tracing algorithm in an iteration pro- 
cedure (Zhang et al., 1994). 
  
Attitude data 
  
  
  
  
À 
  
Computation of 
orientation parameters 
by least squares adjustment 
| 
Calculation of 
temporary terrain coordinates 
for ground control points 
using equations of collinearity 
| 
Calculation of 
polynomial coefficients for the transformation 
from temporary to geodetic coordinates 
by least squares adjustment 
using ground control points 
(and tie points for mosaics) 
] 
  
  
  
  
  
  
  
  
Loop of all input pixels 
| 
Calculation of 
temporary terrain coordinates 
using equations of collinearity 
and actual DTM information 
| 
Transformation into a 
geodetic coordinate system 
using polynomial equations 
| 
Interpolate new DTM value 
  
  
  
  
  
  
  
  
  
  
       
  
  
(actual - new DTM value) 
>eps? 
  
Final geodetic coordinates 
  
  
  
  
  
  
  
Image data 
  
  
  
  
  
Y Y 
  
Resampling 
of the final orthoimage 
  
  
  
  
À 
  
Orthoimage 
  
  
  
Figure 3: The »GASIS« approach 
for the geometric correction of airborne scanner data 
The result of these calculations is a set of data where all 
image points (pixels) are converted to temporary ground 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
points. They are transformed to the »true surface« using 
polynomial equations, which can be derived by only a few 
ground control points. These polynomial equations are 
necessary in order to correct the low frequency trends 
that may be inherent in the original measurements of the 
flight attitude parameters. However, the resulting »true« 
positions form an irregular pattern. In order to derive a 
regular grid of pixels, as it is necessary to generate the 
final orthoimage, appropriate interpolation algorithms are 
applied for resampling. 
In many cases it is necessary to combine adjacent strips 
to an image mosaic. This is achieved by the simultaneous 
determination of the coefficients of all polynomial equa- 
tions in a least squares adjustment. This procedure uses 
not only ground control but also additional tie points, thus 
reducing again the number of necessary ground control 
point measurements. 
The principle of the approach is outlined in Fig. 3. Al- 
though up to now only data of the DAEDALUS scanner have 
been processed with GASIS. The reason for this is, that 
only for this system, which is operated by the German 
Aerospace Research Establishment (DLR) in 
Oberpfaffenhofen and equipped with a special flight 
attitude measurement system, image data and precisely 
specified attitude data have been provided. This data was 
flown in the context of several research projects. But in 
principle the developed approach and its proved functions 
can be applied to any other type of airborne line-scanner 
data. 
The only modules, which have to be adapted to new types 
of data, are the interfaces to the image and attitude data 
formats as well as the derivation of the classical orien- 
tation parameters from these recorded flight attitude data. 
This procedure, however, is in any case one of the most 
important processing steps. 
5. FURTHER DEVELOPMENTS 
In order to fulfil common requirements for the processing 
of airborne line-scanner imagery, the main extentions of 
GASIS will be: 
* the generation of interfaces to data formats of other 
line-scanner data and their associated flight attitude 
data, especially GPS and DGPS, 
e the development of a general module for the 
derivation of the classical orientation parameters from 
any recorded flight attitude data, 
* the integration of powerful radiometric mosaicking 
techniques, 
* the generation of standard file exchange formats. 
Allthough a standard of input data can not be expected in 
the near future in scanner technologies it should be dis- 
cussed to define an experimental data standard for line- 
scanner raw-data which includes the full performance of 
image and attitude data. This aspect will become still 
more important with respect to the developments in digital 
aerial photogrammetry. 
In addition to the desribed approach an other solution of 
the geometric correction problem is under discussion. 
While at the moment orientation parameters are calcu- 
    
    
    
  
    
    
   
   
    
   
   
     
   
    
     
    
    
    
  
    
   
   
    
     
    
     
    
  
  
  
  
   
   
    
  
  
    
    
     
    
   
   
    
    
lated from 
carried out, 
data as ob: 
a result of 
could be d 
be improve 
Zhang, W. 
from Airboi 
meters. In 
Remote Se 
945-950. 
Zhang, W. 
borne Line 
In: Procee 
Sensing C 
pp. 11-447 -