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information about the sensor orbit." At this time this is the 
only information for this model. All the following are the 
conclusions from use of the model. 
In the case of SPOT-HRS data the software reads the data as 
SPOT-HRG, which is understandable because HRS images 
are not a commercial product. Fortunately, the metadata 
format for the HRS data is the same as HRG data thus the 
navigation data are written and also computed correctly (The 
interpolated position and the velocity vectors for the centre 
line are compared with the values which are computed in UCL 
model and they almost identical). Also all the inner orientation 
parameters which can not be changed by the user are the same 
except of one: The principal distance (focal length). The HRS 
principal distance is 580mm while the HRG principal distance 
is 1082mm. However, this discrepancy does not affect the 
accuracy of solution nor, moreover, the accuracy of the DEM, 
as will be shown later. Unfortunately it is not possible to solve 
the model in a geocentric or in an inertial coordinate system. 
The coordinate system should be geodetic (geographic is 
possible but not convenient). 
In order to check and compare the accuracy of the direct 
solution of LPS orbital model with the UCL model the same 
check points as in UCL model are used . The results are 
shown in table 5. 
  
  
  
  
  
  
  
  
  
  
ICP Dx(m) Dy(m) Dh(m) 
Min -18.93 -0.15 -5.45 
Max 19.39 20.51 7,35 
MEAN -4.28 10.75 1.98 
RMSE 9.64 10.99 3.35 
  
Table 5. Accuracy of LPS solution 
The following comments should be made: 
* The UCL model provides us with almost the same 
accurate results in the xy plane, but we should mention 
here that in the LPS model there is an important 
systematic error especially in y-axis. However it should 
be mentioned that the rotation angles in UCL model are 
computed using GCPs and if these GCPs are good, this 
could provide a better solution. 
» The rmse in heights is smaller in the LPS model. 
However, having in mind that the HRG principal 
distance is used instead of HRS, it is not expected to have 
such good results. 
* [tis not clear why we are able to obtain such a good 
result with the incorrect principal distance. This may be 
due to the way in which the LPS sensor model works, 
for example that: 
* The principal distance is used as an initial value in a 
self calibration process where the correct principal 
distance is computed, although it is very difficult to 
do this without GCPs 
* The principal distance is not used. A direct 
transformation is computed between the object space 
coordinate system and the image coordinate system as 
it is defined in the specifications of this model. 
5.3 DEM using LPS 
5.3.1. Introduction. In the LPS all the strategy parameters 
can be changed adaptively, which may improve the results of 
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part Bl. Istanbul 2004 
the strategy application. Adaptive changes take place between 
iterative pyramid layer processing. 
In order to check the accuracy of the produced DEM the 
following sources are used: 
* The Ground Control Points 
+ The IGN 50m DEM 
+ The UCL PITKIN 30m DEM 
The sensor model which was used in order to give reference to 
DEMS is the direct model without any Ground Control Points. 
The model is solved using the information from the metadata 
file. The grid size of the DEMs is square (20mx20m) having 
in mind that all the software can handle DEMs with only 
square pixels. : 
  
Figure 2. Extent of DEMs produced from HRS data 
5.3.2. DEM quality. For the HRS data a lot of tests have been 
made in order to define the best strategy. The DEM with the 
best accuracy and detail was produced with the following 
strategy: 
«;- Search Size: 5 x5 
* Correlation Size: 3x 3 
* . Coefficient Limit: 0.85 
+ Topographic Type: Mountains 
« Object Type: Forest 
+ - DTM Filtering: Moderate 
The search size and correlation size was allowed to change 
adaptively. 
The general mass point quality is described by reference to a 
DEM covering the west area of the IGN reference DEM 
(DEMI in red borders in image 1), which covers almost half 
of the area that HRS data covers. 92% of the points were of 
excellent or good quality, 8% were suspicious. 
5.3.3. DEM accuracy. Accuracy of DEMI. The accuracy of 
the DEMI covering an area of 1509.12 sq. km is described in 
table 6. 
  
   
   
   
  
   
   
  
  
  
  
    
    
  
   
   
   
  
    
   
   
    
  
    
  
  
  
  
  
  
  
  
  
  
  
   
      
   
    
   
    
   
   
   
   
    
    
   
   
   
  
   
  
  
  
  
  
  
  
  
  
   
   
    
    
    
    
  
  
  
  
  
  
Min diff -262.39m 
Max diff 286.35m 
Mean diff -0.48m 
RMSE 16.16m 
  
     
    
   
  
413 
Table 6. Accuracy of DEMI 
The ICP accuracy information is in table 7.