International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
different to that of the Melbourne images. The San Diego area 
has a much more homogeneous texture since it is almost 
entirely forested. Conversely, the Melbourne scene is an urban 
area with a more heterogeneous texture and perhaps more 
ideally suited to image matching. 
6.2 Comparison with reference data 
Although an analysis of the correlation coefficients can be 
instructive, it does not reveal sufficient information by which to 
assess the success of the matching algorithm and the quality of 
the matched points: it is entirely possible that some of the well 
matched points with correlation coefficients greater than 0.8 
may be blunders. Further conclusions can be inferred by 
triangulating the matched points to give object space 
coordinates (using the affine model), and comparing those 
coordinates with a reference digital elevation model (DEM). 
This process was carried out with the Melbourne data where a 
high quality reference DEM was available. This DEM was 
created from aerial photography and resampled to a grid 
spacing of 25m. Significant manual post-processing was carried 
out on this DEM to ensure it gave an accurate representation of 
terrain heights. As a result, it would be expected that points 
triangulated from the matched Ikonos points would be 
uniformly higher, since they represent the surface of the objects 
in the imagery, and not the terrain. 
  
  
  
  
Geometrie Matched points with correlation 
constraitit coefficient > 0.8 
Mean (m) Std. Dev. (m) 
Epipolar 3.41 6.80 
Affine 3.64 7.60 
  
  
  
Table 3. Differences between triangulated points and reference 
DEM - Melbourne Test Area 
As expected, it can be seen from table 3 that the differences 
between the triangulated points and the reference DEM are all 
positive, and of a magnitude that could realistically represent 
the mean building heights within the test areas. Also 
noteworthy is the fact that the differences between the affine 
constraint results and the epipolar constraint results are small, 
although the surface created from the points matched using the 
affine constraint is marginally higher than the surface created 
from the epipolar constraint. 
Since no reference DEM was available for the San Diego test 
area, a similar comparison was not possible. 
6.3 Visual analysis 
Finally a visual analysis of the results was carried out. A 
surface model was created for each test area from the 
triangulated points. Figure 3 shows a 5m grid DSM of the 
Melbourne test area, while figure 4 shows a similar DSM of the 
San Diego test area. In each figure the lighter tones of grey 
represent high elevation, whilst the darker tone represent low 
elevation. 
  
  
Figure 3. DSM of Melbourne test area 
Figure 3 clearly illustrates the potential of matching stereopairs 
of Ikonos images. The resulting DSM shows many features, 
including buildings, vegetation, bridges, overpasses and even 
roads and railways. Although some blunders are apparent, 
particularly in the river in the eastern part of the test area, the 
majority of the matching has been successful. Even with the 
presence of the surface features, it is still possible to recognise 
the pattern of the underlying terrain, with areas of high ground 
in the northern part of the test area. Note that this image has 
been shown from a near-nadir perspective due to the fact the 
terrain varies very little across the image. 
  
Figure 4. DSM of the San Diego test area 
Figure 4 shows the San Diego DSM displayed in perspective 
view (with the vertical scale exaggerated). It is clear to the 
observer that the DSM is a very good representation of the 
underlying terrain, with hydrological and topographic features 
standing out very clearly. There are very few, if any, obvious 
blunders. 
7. DISCUSSION 
This paper has presented the use of two different geometric 
constraints on two stereopairs of high resolution satellite 
imagery. The results of the matching procedures have been 
analysed by assessing the correlation coefficients, triangulating 
   
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