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The hardware is represented in Fig.1. All of the stereo-plotters were 
equipped with shaft encoders, which are connected to the PRIME 400 
computer by a microprocessor-controlled interface. The connection is an 
ordinary terminal line (current loop/RS 232). The counter in the inter 
face can be controlled and recalled by a set of simple commands (alpha- 
numerical strings). 
All of the programs run on the central computer and can be recalled 
interactively on the terminal assigned to each stereo-plotter. 
3.2 Mathematical model 
3-2.1 Concept 
The mathematics of the simultaneous analytical determination of all 
orientation elements has been discussed by various authors (Schmid 
1979), (Schwidefsky and Ackermann 1976). We decided to use the 
analytical treatment of the bundle method formulated in the geodetic 
object coordinate system, as this seemed the most elegant approach to 
this problem. The method is based on the principle of central 
perspective, which is expressed by one single condition, the condition 
of collinearity. 
In the literature of analytical photogrammetry the following case is 
usually described (Schwidefsky and Ackermann 1976): 
Image coordinates of a number of control points are measured by a stereo 
or mono comparator. Using the geodetic coordinates of selected control- 
points, the 12 orientation elements and the coordinates of all control- 
points are computed in one simultaneous least-squares adjustment. The 
left and right image coordinates of one and the same control-point are 
usually treated as independent observations or more precisely as 
uncorrelated random variables with normal distribution. 
Since we are actually measuring geometrical quantities with the analog 
stereo-restitution instrument rather than image coordinates, a slight 
modification of the usual mathematical model of the bundle approach was 
sought which suits the characteristics of the analog instrument better. 
In our case, the four geometrical quantities which are actually 
recorded, 3 model coordinates and a fourth value which is a measure for 
the y-parallax, are treated as observations. We call this set of 
observed quantities 'pseudo-model coordinates' because these values do 
not correspond to real model points. Residuals at these variables, 
rather than at the image coordinates, are estimated in the least-squares 
adjustment. This way the left and right image coordinates of homologous 
rays are treated as statistically correlated variables. 
3*2.2 Observations 
The geodetic coordinates (U.V.W)^ ; i=1,2,...n^ of the absolute control- 
points are introduced as observations with known variance. 
The analog instruments are used in the following special mode 
('stereo-comparator mode'):