sponding projected images of the two photos into coinci- 
dence. The images parallel to the air base are brought into 
coincidence in the x-direction at strategic locations by 
moving the tracing table measuring- or floating-mark up or 
down with a thumb wheel. Displacements in the y-direction 
(y-parallaxes) are removed at these locations by adjusting. 
appropriate screws on the projectors. This parallax-clearing 
procedure is repeated around the stereomodel until the 
entire area is "cleared" of y-parallax. 
The resulting stereomodel formed from the two projected 
photos is then fitted to control points by changing the dis- 
tance between the projectors to adjust the stereomodel 
scale and by tilting and rotating the relatively-oriented 
projectors as a unit. Once oriented, the projectors remain 
fixed throughout compilation, which in figure 1l, involves 
viewing and measuring the 3-dimensional imagery projected 
onto the platen of the tracing table. Measurements are made 
by means of the floating mark, a tiny light that can be 
moved in any direction from image point to image point 
within the stereomodel. Features are delineated with a 
pencil fixed directly below the floating mark. 
With analytical stereoplotters, relative and absolute 
orientation is accomplished by procedures similar to those 
used with analog instruments, but the relative and absolute 
orientation elements are determined mathematically from the 
x- and y-measurements made at each parallax-clearing point. 
In computing relative and absolute orientations, three 
angles and the x-, y-, and z-coordinates for each photo of a 
stereopair are determined. 
The plates are not physically oriented in an analytical 
stereoplotter, but instead, a mathematical stereomodel is 
formed, and the angular and positional orientation elements 
are expressed in terms of x- and y-shifts of the plates 
needed to clear the circle of imagery aroünd the floating 
mark. As the floating mark in the viewer is moved across the 
stereomodel by handwheels, or whatever, the plates are con- 
tinuously shifted in x and y, so that the local area viewed 
and the floating mark position in the imagery are the same 
as if the photographs had been physically oriented and a 
stereomodel had been formed and fitted to control by the 
perspective projection of the photographs. The incremental 
x- and y-shifts of the plates are so small and smooth that 
the operator has the illusion that the floating mark is 
traversing a fully cleared stereomodel. Since the orienta- 
tion of the photos and the position of the floating mark is 
expressed in terms of x- and y-shifts, a means is provided 
for correcting for any stereomodel distortions that can be 
expressed in terms of adjustments to the x- and y-shifts. 
Advantag es 
Some of the advantages of analytical stereoplotters over 
conventional optical-mechanical stereoplotters are: