JUNE, 1964 
143 
The Stereo Line Plotter 
The left side of the instrument is a plate with parallel scribed lines that can be moved in 
the ^-direction. Light passes through these parallel lines to the back of the left photograph. 
The left side then is the height-measuring part of the instrument and simulates a parallax bar. 
The observer viewing through a stereoscope sees a three-dimensional model of the terrain 
and a plane of parallel lines either above, through, or below the level of the terrain. 
All movements for relative and absolute orientation, or in effect a combination of the two, 
are carried out by moving only the right projector, which projects a set of parallel lines on the 
back of the right-hand photograph. 
Since the tilted photographs are laid flat, we must accept the height deformations of the 
stereo model as illustrated in Figure 1 and we have no choice but to deform the measuring surface, 
that is the plane of lines, to fit the deformed surface. By moving the plate under the left picture, 
we can then raise or lower this plane of lines and in effect get an intersection of this plane with 
the stereo model. It is then a simple matter for the topographer to trace contours on the photo 
graph that represent the intersections of this plane of lines and the stereo model. 
The deformation of the plane of lines to compensate for model deformation due to tilt, 
tip, etc., is explained and described in the following paragraphs. 
If the right projector in Figure 2 is rotated about the z-axis, the x-parallax between the lines 
in the right image is increased or decreased. The change in parallax will tilt the plane of lines 
in the cross base or to direction and will compensate for the model deformation due to yZ // 
(Ak + ll/f i/to") and 12, the absolute value of cross tilt in relation to ground control. 
If the right projector is moved up toward the right-hand photograph, the parallax between 
the corresponding lines on the left image and those on the right will decrease in a near-linear 
function, causing the plane of lines to slope up from left to right along the air base. A downward 
movement of the right projector has the opposite effect. These movements compensate for 
xZ/b'f(Abz — 2l/d4>") and <f», the tip of the model in relation to ground control. 
The expression —x^Z/b'f is the so-called 0 cylinder and can be compensated by rotating 
the right projector about they-axis. The Aco or differential-tilt error can be rectified by a modi 
fication in the design of the present instrument, but since this design change would add to the 
cost of the equipment and detract from the simplicity of operation, it was not incorporated into 
the present instrument. The stereo model will be divided along the diagonal and contoured in 
two parts with different height settings to compensate for the height deformation due to differ 
ential tilt. This is not an exact solution but should give satisfactory results. 
Figures 3 to 6 illustrate the present model and the component parts. 
One of the attractive features of this instrument is that it can be built for the most part from 
existing material. For instance, the projector lens is a one-inch-focal-length 35-mm projector 
lens that costs $23. The other parts are copper pipe, electrical wiring, simple gears, wood, a 
standard nine-by-nine-inch glass plate, and a multiplex plate. The only parts of the instrument 
that require precision tooling are the x-motion on the left plate and the up-and-down motion of 
the right camera. It is truly a poor man’s plotter; it can be built by skilled technicians and we 
believe the cost will be reasonable. 
The instrument has a number of features that add to the initial cost but should prove to 
be a saving in the long run. For instance, the projector lamps used can be purchased in any 
photographic store, but they have a coated heat-resisting surface on the top and should stand 
vertical for long life. Rather than design a new light bulb, we used a combination of copper 
tubing and mirrors to transmit the light from the upright bulb around a 90-degree turn to the 
right-hand camera. The large box under the left plate holds a mirror set at 45-degrees to transmit