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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
  
of ongoing developments involving the affine model, two 
principal issues have been dealt with. The first concerns the 
impact of a height correction for image coordinates, which 
arises from the discrepancy between an affine and central- 
perspective projection. The second relates to the potentially 
adverse effects on the standard affine model of dynamic re- 
orientation of the satellite during image recording. These effects 
can, however, be modelled to reasonable accuracy via time- 
variant affine parameters, along with four quadratic additional 
parameters and the recommendation that the object space 
coordinates be in a projection system such as UTM. 
Finally, experimental application of affine bundle adjustments 
to one IKONOS and two QuickBird stereo image pairs have 
illustrated both the advantages and potential shortcomings of 
the affine model, in both its standard and extended forms. The 
results from the IKONOS Hobart testfield have shown that the 
affine model does not cause significant accuracy degradation, 
even over terrain with an elevation range of 1300m. Moreover, 
while the perspective-to-affine image correction procedure 
leads to accuracy improvements, these amounted in Hobart to 
only about 0.2 pixels in the across-track direction. 
On the other hand, it was found for QuickBird Basic imagery 
that the extended formulation of the affine model with four 
additional parameters, Equation 7, was needed to produce sub- 
pixel accuracy. This also required 10 to 15 GCPs. The 
accuracy achieved from the experiments overall is essentially 
equivalent to that obtained with the more rigorous RPC model 
(eg Fraser & Hanley, 2004; Noguchi et al, 2004). These 
encouraging results lend weight to the view that the affine 
model is a practical sensor orientation model that should, 
however, be used with some caution for QuickBird Basic and 
IKONOS Forward scanned imagery. 
6. ACKNOWLEDGEMENTS 
The authors wish to thank Digital Globe, Space Imaging and the 
Geographical Survey Institute of Japan, respectively, for 
providing the HRSI stereo pairs of Hobart, Melbourne and 
Yokosuka. 
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