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Clarke, Tim 
  
  
Figure 2. Configuration of measurement system with respect to rib and spar. 
5. 3-D ESTIMATION 
To set the system up two steps are required. In the first the cameras are calibrated and in the second the relative orientation 
of the cameras is estimated. To calibrate the cameras a 3-D calibration artifact is used. The artifact consists of a number of 
posts mounted on a metal base. The design of the artifact is such that when viewed from an angle the minimum number of 
targets are occluded by the posts. The artifact has a significant element of 3-D to enable the highest possible accuracy in the 
determination of the relative orientation of the 
  
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Figure 3. The calibration and relative orientation artifact 
being used to calibrate the cameras 
The calibration process normally involves making the robot rotate the measurement head to observe pairs of images of the 
artifact from four different directions followed by a four further images with the equivalent of a camera roll of 90 degrees. 
Typical image residuals for the small format cameras used are typically 1/30 of a pixel. The 3-D estimates of the targets are 
stored and then used to compute the relative orientation of the cameras. 3-D estimation is then by direct intersection. The 
small format cameras are used to develop the system and will be replaced by higher resolution sensors at a later stage. 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000. 139