226 
- The bundles of rays are recovered in the conventional way by adjusting 
the images into the fiducial-mark system and setting the calibrated 
focal length c. 
- An approximate base length b x is set (bY=b z =0). 
- All angular scales are set exactly to zero: (0 1 = ^ = K 1 = (0 2 =(p 2 = K 2 = 0 
In this mode the analog instrument simulates a stereo-comparator. The 
following geometrical quantities are measured and recorded for each 
control-point by the shaft encoders: 
- Coordinates in the model coordinate system (X,Y,Z) 
- A fourth value bY2 (WILD A7) or b^ (WILD A8) 
The measuring and recording procedure is described as follows (Fig.2). 
The operator tries to intersect the two homologous rays of each control- 
point by varying bYo or b W 2 an< * 2. He finally reaches a situation in 
which the two collinear points and P2 are separated only by a very 
small distance d. The floating mark is now set as closely as possible 
to the medium position P. Finally, the pseudo-model coordinate values 
of the floating mark are measured and recorded on disc. 
Measuring errors have been taken into consideration for the four encoder 
readings X,Y,Z and b W 2 or bf 2 - Their a priori variance depends on the 
increment of the encoder and also reflects the mechanical imperfections 
of the lead screw system. 
In our model the homologous rays r^ and iv, are not constrained to 
intersect in the model space. The residual separation d is treated as 
another measuring error and is minimized in the least-squares sense. 
Seven residual components are added to the four measured quantities 
(X,Y,Z and bYp or b (1) p) of each control-point in order to get the 
X 2 = % + v x - 
z 2 - z m + V Z “ V Z 
d bY 2 = d bY 2 + ^bY 2 
2 |v| + v| + v| reflects the remaining 
adjusted pseudo-model coordinates: 
II 
o<~ 
X M + 
<> 
+ 
% 
II 
<>r 
Y M + 
v Y 
+ 
v Y 
n 
Z M + 
^Z 
+ 
% 
cL 
b (D2 
= cL + 
d (02 
V. 
d C02 
The residual separation: d = 
error of the parallax measurement.