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| COMMISSION X - X St 
| ^e pendix te the INT. ARCHIVES © QF 
| PHOTOGRAMMETRY, Vot. XIk«, 1956 | 
Considerations on Photogrammetric Plotting Instruments Utilizing 
Mechanical Projection 
by M. SANTONI. 
As is known to all concerned with Photogrammetry, the principle of the mechanical 
projection of the lines of sight was applied since the earliest practical developments in 
this science (among others by Laussedat in France). The simplest application of this 
method can be considered the use of rulers to trace on a the sheet horizontal projections of 
the lines of sight, in order to determine from the intersections the planimetric positions 
of the points to be plotted. 
The basis for a more complete mechanization of the method, extensible to height deter- 
mination was proposed by the “graphie sectors” developed by Eng. Paganini of the Italian 
Military Geographic Institute. 
Basic principle of such methods was always the measurement on the photographs of 
the x y coordinates of the image-points with respect to the perspective principal point, 
or more exactly, with respect to the axes of the photograph. These values, referred to 
the objective focal length, permitted to reconstruct the directions of the lines of sight for 
the considered points. 
A very important step toward easier identification of the image-points corresponding 
to an object-point, not always readily identifiable because of the two different points of 
observation, was the application of stereoscopic vision, made possible by the development 
of Pulfrich's Stereocomparator. 
In addition to providing easier identification, the new instrument marked substantial 
improvement in the accuracy of parallactic measurements that reached soon to the 
present standards (within 0.01 mm). 
Equally known to students and researchers in the photogrammetric field is that i 
the first decade of this century Von Orel devised a successful combination between 
mechanical projection of the lines of sight (where, as already mentioned, pioneer work 
had been done by Paganini) and the stereoscopic measurement allowed by Pulfrich's 
stereocomparator. Very good plotters, the Stereo-autographs, were manufactured by 
Zeiss for utilization with ground photographs taken exclusively by horizontal axis. 
Accuracy attainable in these instruments was however strictly dependent on the 
basic assumption that the second nodal point of the taking objective, from which the 
focal length was measured, was to be considered the sole geometric center of projection 
of the whole perspective. In other words, it was assumed that only one second node 
existed and, therefore, that there was no distortion of the image. 
Actually, as the lens apertures were extremely moderate (1:10 —1 : 20), the angular 
field allowed by the cameras in use at the time very narrow and appropriate- 
ly calibrated focal length value was chosen for plotting, the above geometrical conditions 
might be considered fulfilled to a point. 
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Aerial photogrammetry was however soon tc appear, proposing its wider range of 
possibilities as well as its harder problems. For the exposures, necessarily instantaneous, 
it was necessary to use brighter objectives, in which however distortion reached so high 
values as to become troublesome. On the other hand, it was impossible to impose exterior 
orientation elements during the exposure, which prevented the use of Orel-Zeiss plotters 
as only suitable to reproduce from particular exposure conditions. 
New plotting instruments and methods had therefore to be devised in order to meet 
the new circumstances. It was the time of both the first developments of optical-mechanical 
projection of the homologous rays, and optical projection of the photographs. 
In both cases, the plotting instrument utilized two projectors, optically equivalent of 
the taking camera. Orthoscopic defects i.e. distortion, were eliminated, at least in prin- 
ciple, in the optical path inversed of exposure, applied in the plotting. A totally mecha-