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being added for the new row in the array, and subtracted 
for the row leaving the sub-array being compared. This 
results in a drastic savings of time for the computation 
of the coefficients in the search array. Figure 1 shows 
the path of the target array as it is lagged through the 
search array. 
target 
array 
search 
array 
Figure 1. Path of target array within search array. 
FIDUCIAL LOCATION AND SCANNER CALIBRATION 
Coordinates of match points obtained in the scanner system 
must be related to the fiducial axis system in order to 
perform photogrammetric calculations. First the fiducial 
marks must be located in the digitized image and the 
coordinates refined to sub-pixel magnitudes. The refine 
ment procedure is described later in the section on control 
panel recognition. After the scanner coordinates are 
determined an eight-parameter transformation is performed 
to relate the two systems. The coefficients of this trans 
formation can then be applied to other points to generate 
photo coordinates. The formulas used are given in 
equations 2 and 3 below. Several transformations were 
tested, but these equations gave the best results with a 
minimum of computational effort. In the equations, the A's 
and B's are the transformation coefficients, X, Y are the 
row column coordinates of the scanner, and x,y are the 
calibrated fidicual coordinates. 
x = A i + A 2 *X + A 3 *Y + A 4 *X*Y (2) 
y = B 1 + B 2 *X + B 3 *Y + B 4 *X*Y (3) 
This transformation partially corrects for scanner distor 
tions by forcing a best fit to the fiducials, however some 
distortions still remain. Causes of these distortions in 
clude eccentricity of the drum axis, film warpage introduced 
due to stresses exerted at the mounting clips, bulging of 
the film due to centripetal acceleration of the drum, and 
nonuniform drum rotation and lead screw advancement. Use 
of a more precise flat bed scanner would eliminate most of 
these distortions. However, most of the distortion is 
systematic and could be corrected for.