STEREOPHOTOGRAMMETRY. In the earliest appli cations of photography to surveying, use was made of various simple graphical constructions or calculations in order to deter mine bearings and angles of elevation or depression from the camera station. In order to simplify these constructions or calculations, the photograph was usually exposed in a vertical plane. Two photographs, taken from the ends of a known base and treated in this manner, thus enabled a complete survey Jo be made of all visible ground in the area common to the pair. The necessity for avoiding acute graphical intersections in the fixing of points usually meant a long base, with a resulting diffi culty in the identification of corresponding features in the two widely dissimilar views. The next step was consequently to return to a comparatively short base and at the same time to eliminate the indetermination of acute graphical intersection by the parallactic measurement of distances.
The Stereocomparator.—The first practical application of this method appears to have been in the stereocomparator of Tourcade in 1900. The two photographs were taken in the same vertical plane, although not necessarily at the same height, and were of course placed in the same plane in the comparator. Bear ings and elevations were still calculated from the photo-co ordinates of points on one of the pictures, while the distances from the base were calculated from a stereoscopic measure of parallax. The latter was effected by a lateral shift of one plate relative to the other, until the stereoscopically reconstructed image of the setting feature appeared to lie at the same distance from the observer as the fused image of a pair of floating marks contained in the eye-pieces of a binocular microscope used for the examination of the plates.
The stereocomparator has had an extensive application in the past, and is even now directly applied in one modern method of survey from vertical air photographs. From the practical point
of view, however, its initial use suffered from two serious draw backs, in that it required the computation of space co-ordinates from independent measurements, and that it implied a system of point by point plotting. (See also BINOCULAR INSTRUMENT: The Stereoscope.) The Thompson Stereoplotter.—The first of these disadvantages was overcome in the 1908 model of the Thompson stereoplotter, which may consequently be considered as the first example of the true plotting machine. It employed the stereocomparator principle without modification, but by the addition of a lever system.
The Stereoautograph.—The disadvantage of point by point plotting remained in the Thompson machine and was overcome by the von Orel stereoautograph of 1909–i I. It is interesting to note, however, that Thompson in 1908 had in view the con struction of an automatic machine called the "stereoplanigraph." This would no doubt have been on the same general lines as the stereoautograph had it reached maturity. The stereocomparator is again used in the von Orel machine, but the plotting system is based on intersections from the ends of the base rather than on an explicit measure of distances. Reduced to its lowest terms, it consists of two azimuth levers which are coupled to the lateral shift of the two plates. This simple arrangement would, however, introduce mechanical difficulties in the carriage of a drawing pencil at the variable intersection of the two levers, and in the necessity for shifting the pivots of these levers in accordance with variations in base length.
Suppose, in the first alternative, that LP, RP (fig. I) were the azi muth levers ; P being the plotted point and LR the base. Then if any two parallelograms LPQR', RPP'R' are drawn on the rays