Helen Burman 
  
r To solve for all misalignment angles only by measuring differences between strips, at least three strips flown in 
different directions are needed. 
3.1 Recommendations 
To get redundancy in the measurements, it is advised to have four strips crossing each other and to measure elevation or 
intensity differences at the borders of the strips (figure 3). 
  
iq] Mw Mv e 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
..| Matching in 
(SA | area with 
1 e m^ gradients 
e 
L3 
! EE 
| mn m 
LS} 
NT 
  
  
  
Figure 3 Recommended configuration for the alignment procedure. 
4 MATCHING PROCEDURE 
0 
Interest points to be used for matching are chosen by searching for points with large gradients, either in one or both (X 
and Y) directions. This is to find points, corners or edges. For this, approximate values one grid of height and one grid 
n of intensity values are calculated from the irregular laser data by using the finite element method. Large gradients are 
e 
found by using the Sobel filter. Within a 7x7 window, the square sum of the gradients in X and Y are calculated and the 
final weight is calculated as the Root Square of these values. 
  
ve 5361526.) > 
j=l i=l j=l i=l 
W weight of interest 
qx; grid point (i,j) Sobel-filtered in X-direction 
qvi, grid point (ij) Sobel-filtered in Y-direction 
  
International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B3. Amsterdam 2000. 129