Helen Burman 
  
If there are ground control points available, they will be introduced as additional observations of the elevation or 
intensity grid. 
3 THE ALIGNMENT PROBLEM 
In the alignment problem, the rotation between the INS and the laser scanner co-ordinate system is to be determined. In 
this chapter, a number of different test configurations will be discussed in order to define an optimal alignment set-up. 
Studying the observation equations one can see that there are linear dependencies between the unknowns. Æ.g. a shift 
along the flying direction can be explained by a positioning or by a pitch error. À shift across the flying direction can be 
explained by a position or by a roll error. Therefore, the datum error has to be known or be the same for the whole 
alignment flight. When no ground control is used, the datum errors are assumed to be zero and this is included in the 
adjustment by additional observations with high weights. 
If there are no gradients (flat area), only the roll misalignment can be solved by measuring the differences in elevation 
between two strips (figure 1). 
  
Nf 
Differences in elevation 
  
  
  
  
  
  
  
Figure 1 Misalignment in roll causes discrepancies between strip also in flat areas. 
If there are ground control points, all misalignment angles can be solved but two strips have to be flown to be able to 
separate datum errors from misalignment, providing there are gradients in elevation or intensity. 
If no ground control is used, the only observations are differences between the strips. As mentioned before, the datum 
errors are then assumed to be zero. Two strips flown in different directions will cause linear dependencies between the 
three angles (figure 2). 
  
  
Discrepancy caused by 
heading or pitch error 
/l 
[| 
33 
  
  
  
  
  
Ts 
  
  
  
Figure 2 Example of linear dependency between misalignment angles. 
  
128 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000.