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Martin Smith 
  
1.2 Absolute and Exterior Orientation 
Two of the most difficult photogrammetric tasks to automate are absolute and exterior orientation. This is because 
traditionally ground control points are traditionally natural pieces of detail which are difficult to record in a convenient 
and suitable computer compatible form. Pre-marked targets on the ground can be recorded as a ‘target’ master pattern 
in a computer and used in automatic matching with the corresponding ‘target’ patch in the digital image. Researchers 
have investigated the use of specific features such as manhole covers or existing maps or CAD models as ‘targets’ 
(Schickler, 1992; Drewniok and Rohr, 1997a 1997b; Morgado and Dowman, 1997; Dowman, 1998) with some success. 
However these tend to be solutions when very specific ground data is available. What is required is a technique that is 
more general, and where the ground coordinate data is easy to obtain. 
13 In-flight use of the Global Positioning System (GPS) and Inertial Measurement Unit (IMU) 
The use of in-flight control in the form of GPS and IMU is a practical reality. Operational systems are available and 
research is being undertaken to realize the full potential that can be achieved. This will play a significant role in 
providing control for images in the future. The aim is to enable the parameters of exterior orientation of a photograph to 
be directly measured and introduced into a photogrammetric plotting instrument or workstation. This technology is still 
relatively new and not widely used at present. Therefore there is still a considerable amount of aerial photography taken 
without direct measurement of position and attitude. Besides this photography, there is obviously a vast archive of 
photography available around the world which similarly does not have any in-flight control associated with it. This 
archive photography is often the photography that the *non-specialist user of photogrammetry might be interested to 
use. So there is still a need for an easy form of indirect image orientation. 
1.4 The aim of a new approach to absolute and exterior orientation 
The aim of any new approach should consider the following points:- 
e it needs to be easy to use; 
e the ground control must be easy to obtain; 
e it needs to have the potential for automation; 
e it needs to be flexible and suitable for analytical and digital techniques so it has the widest possible application. 
2 ANEW APPROACH TO ABSOLUTE AND EXTERIOR ORIENTATION 
After consideration of the above points in section 1.4 and studying readily available features on aerial photography that 
might be suitable for ground control, a number of conclusions were drawn. It was realized that it would be difficult to 
find a small target in any image that was very ‘cluttered’ with detail. Premarking would perhaps be helpful with new 
photography but of no use with archive photographs. Detailed selection of traditional ground control points of natural 
features is not sufficiently easy as it requires too much skill and time. Finally it was decided to investigate the use of 
linear (or curvilinear) features as ground control as these were easily visible on the image and often accessible on the 
ground. This also had the advantage that a number of researchers were investigating, with some success the automatic 
extraction of linear features from aerial and satellite images for example, Gruen et al. (1995, 1997). The ways of 
obtaining ‘real world’ (ground) coordinates of the linear features are numerous. These could be available from an 
existing map or data base. With the increasing availability of GPS, the curves could be obtained by an individual 
walking along the linear feature, in the case of foot paths, or driving along the curves, in the case of a road network. 
Obviously the accuracy might be expected to reduce as the clarity of the definition of the curve due to the method of 
data capture (perhaps on a motor vehicle) becomes poorer. The interesting point about linear features is that they take 
many forms, including roads, paths, fence lines, waterways, etc. Also they are frequently visible on a wide variety of 
photographs. It is also possible to measure these features equally well using diapositives in an analytical plotter. 
The concept is that linear features will be extracted from individual photographs or in a stereo model. These features 
could be obtained by manual or automatic means in an analytical plotter or by using digital techniques. The 
corresponding ground curve will be coordinated in three dimensions, by any suitable means (GPS, digital map plus 
DTM, etc.). The curves are then automatically matched to find the ‘best’ fit together. Finally the transformation, three 
dimensional for absolute orientation or perspective transformation for exterior orientation, will be performed from the 
corresponding point coordinates of the matched curves. This project has concentrated on the automatic matching of the 
two curves. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 851