Reference Approximate fiducial Acute Honetol po- | Pose Stm Availability Comments 
positioning sitioning tion 
Kersten, Háring | modified Hough trans- | least squares mat- internal use at swissair | different fiducials described 
manua ; 
(1995) form ching Photo + Surveys Ltd. in terms of features 
implemented in the Soft- ; ; 
grey value correlation, | least squares mat- p n different fiducials managed 
Lue (1995) > : manual Plotter from Vision Inter- . 
hierarchy ching : through fiducial database 
national 
; à different fiducials possible, 
Schickler (1995 Spas AE 1 lati automatic implemented in PHO, automatic pos./ne Be ine 
rrelation u | : ; - 
Clickler 4 ) hierarchy Frey yous co DIS ST from Zeiss 9 : s 
nation 
. | fitting of parabolas to ; 
Strackbein, Hen- | binary image analysis, gob : internal use at the LVA ; J 
: grey value function manual (?) : for circular fiducials only 
ze (1995) no hierarchy ( Nordrhein-Westfalen 
  
  
  
  
Table 1: Approaches to automatic interior orientation 
  
Figure 3: Examples for fiducials causing problems in the 
automatic interior orientation. Top left: poor contrast 
between fiducial and image content, top right: poor con- 
trast between fiducial background and centre, bottom 
left: no fiducial in the green channel, bottom right: scrat- 
ches in the vincinity of a fiducial 
4 AUTOMATIC EXTERIOR ORIENTATION 
The exterior orientation describes the transformation 
between the image and the object coordinate system. In 
a mapping project involving more than two images tradi- 
tionally an aerial triangulation was carried out for point 
densification, followed by a separate exterior orientation 
for each model, split up into relative and absolute orien- 
tation, the latter being based on the densified points. 
Thus, the exterior orientation was determined twice, be- 
cause (1) the results of aerial triangulation were not 
considered accurate enough, (2) the orientation parame- 
302 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996 
ters could not be used in the plotter due to missing 
interfaces, or (3) sometimes the bundle adjustment was 
carried out using smaller scale imagery than the mapping 
step for economical reasons. 
Today, the situation has changed. The accuracy of aerial 
triangulation has improved, and the interface problems 
have been overcome. As a consequence, the exterior 
orientation parameters determined in the aerial triangu- 
lation are often downloaded directly for stereo plotting 
from single image pairs. Thus, it could be concluded that 
separate solutions for relative and absolute orientation 
are out of date, and what remains to be investigated is 
automatic aerial triangulation (AAT). In fact, various 
authors have addressed AAT, and solutions already exist 
(Tsingas 1992; Ackermann 1995a; Schenk 1995; see also 
reports on matching strategies for point transfer by Forst- 
ner 1995, on practical applications of AAT by Haumann 
1995 and a discussion on commercially available systems 
by Kolbl 1996). However, if one looks at the different 
steps within aerial triangulation, the tasks of identifying 
conjugate features in various images, and of determining 
the position of control information within the images can 
be identified. The first task is linked to relative and the 
second one to absolute orientation. Thus, while AAT is 
the major subject in research and development in image 
orientation today, at a finer level the relative and the 
absolute orientation are still core problems which need 
to be solved. Obviously AAT involves other issues such 
as multi image matching, connecting strips with varying 
degree of side overlap, managing large data volumes etc. 
A detailed discussion of AAT can be found in Schenk 
(1996) and is out of the scope of this paper. 
Before discussing the possibilities and limitations of 
automating the exterior orientation by means of indirect- 
      
       
     
   
        
    
       
    
   
    
  
    
  
  
  
  
  
  
  
  
  
  
   
   
   
   
   
   
   
  
  
    
   
   
   
   
    
   
   
   
    
   
    
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