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7.1 Calgary Test Flights 
A special variant of the casi was developed by Itres 
Research Ltd. for this project. This sensor contains fore, 
nadir and aft look directions giving it stereo capabilities. 
The sensor has 512 pixels along each scanline and a field of 
view of 43.5°. The fore and aft look directions are 16° off 
nadir. These limited off-nadir angles were due to the CCD 
geometry. 
Three flightlines were flown over the city of Calgary. Each 
flight line was 10,000 lines long with an along track pixel 
size of 1.6 metres and an across track pixel size of 2.5 
metres. Included on board the aircraft were the casi sensor, 
a Litton LTN-90 inertial system and an Ashtech P12 
receiver. Another GPS receiver was placed near the centre of 
the test area. GPS positions were processed to 0.50 metre 
accuracy. 
Due to the fairly large pixel size, large area and urban nature 
of the area, placing targets on the ground was impractical. 
Therefore, road intersection centres were used as targets 
instead. These control points are somewhat poor due to 
errors in matching the road intersection centre on the ground 
to the image. However, in a production environment, this 
would be the most likely way control would be established. 
7.2 Test Results 
To test the accuracy of the system, bundle adjustment results 
are shown. These would be equivalent to geocorrection 
accuracies if the DEM was taken into account. Each 
: : b ; : 
adjustment solves for a different dR, for each flight line. 
The values for the orientation differences &, ® and x are 
compared for each flight line. 
Three different solutions are shown for the above mentioned 
data set. They are: 
a) First, the bundle adjustment was run with all control 
points and fore, aft and nadir look directions. This 
gives the optimal solution from all available data and is 
used to compare results. 
b) Next, the bundle adjustment was run with only 4 control 
points and fore, aft and nadir look directions. This 
corresponds to the case of minimum ground control for 
calibration. The remaining control points are included 
as check points. This would be a standard stereo 
Scenario. 
c) Lastly, the bundle adjustment was run with 4 control 
points and only the nadir look direction. The 
remaining control points are included as check points. 
This scenario is would be used with the standard multi- 
spectral casi. 
Test (a) results show that the overall horizontal accuracy is 
about 1.2 pixels (3.0 metres). The largest control point 
error was 5 metres (horizontal). Larger errors in the Z axis 
are due partially to a poorer height geometry as seen by the 
Standard errors, and also due to poor control point 
registration between the image and ground. 
  
  
Control points. 2.778 2.861 14.88 27 
RMS (m) 
  
Check points 
RMS (m) 
  
  
  
  
Image residuals 
(pixels) 
  
  
  
Avg. standard 2.32 2.11 8.60 281 
  
  
  
  
  
  
error (m) 
  
Table 6: Test (a) Results, All Control Points 
Used With Stereo Pushbroom Sensor 
With improved targeting, the control point residuals will be 
reduced as they are currently the largest error source. Other 
error sources result from the GPS and INS, but these are both 
below the sub-pixel level. There are also errors due to the 
calibration, but they are difficult to quantify individually. 
The principal point (=CCD centre) is most difficult to 
calibrate. However, erroneously calibrated parameters will 
be partially compensated by the INS/sensor offset angles. 
  
  
  
  
  
  
  
  
  
0 (deg.) 0 (deg.) K (deg.) 
Flightline 1 1.9697 5.7278 3.1517 
Flightline 2 1.9203 5.5642 3.4284 
Flightline 3 1.9998 5.7948 2.7574 
  
Table 7: INS/Sensor Offset Angles for Test (a) 
The values for the INS/imaging sensor orientation offset 
b 
dR, are shown in Table 7. Although they are relatively 
consistent, there are some variations present. Most 
significant variations are seen in the K (yaw) values. This 
larger variation is expected because the system, with its 
fairly narrow swath angle (43.5°), is less sensitive to yaw. 
Other differences may also be due to the fact that flightlines 
1 and 3 were in one direction and flight line 2 in the 
opposite direction. 
  
  
  
  
  
  
X Y Z no. 
Control points. 1.497 1.401 2.325 4 
RMS (m) 
Check points 2.646 3.701 22.33 23 
RMS (m) 
Image residuals 0.99 0.72 1057 
(pixels) 
Avg. standard 2.43 2.18 301 
error (m) 
  
  
  
  
  
  
Table 8: Test (b )Results, 4 Control Points Used 
With Stereo Pushbroom Sensor 
Table 8 shows results of test (b) using only 4 control points. 
These results are very similar to those of Table 6 which 
means there is no significant accuracy degradation due to 
using fewer control points. Four points are sufficient for 
this size of block. This means that after calibration, the 
georeferencing system performs with the expected accuracy. 
Tables 9 and 10 show results using only the nadir channel. 
This is the geometry of the standard casi sensor. The errors 
in y are slightly larger than from the (a )and (b )tests. 
199