a general-purpose stereophotogrammet-ric instrument designed to make topographic maps using aerial photographs with angles of inclination from the vertical up to 3°. The stereoprojector was proposed by G. V. Romanovskii in 1950 and is designated by the abbreviation SPR (stereoprojector of Romanovskii).

Figure 1 shows a schematic diagram of a stereoprojector. Photographs (1) are always positioned horizontally in the stereoprojector regardless of the angle of inclination from the vertical at which they were taken. The effect of the angle of inclination is compensated by correction mechanisms that shift the lenses (3) by the necessary amount. Marking is done by means of arms (4)

Figure 1. Diagram of a stereoprojector

that rotate around the centers of projection (5) and link the photographs to a coordinatometer. The coordinatometer is represented by guides X, Y, and Z. The base mechanism moves along carriage Z; it consists of the guides and carriages of the base components bx, by, and bz, which are used to set the projection base length. As measurements are made, the photographs are moved relative to fixed marks.

The centers of projection of stereoprojectors do not usually coincide with the centers of projection of the aerial photographs. This causes a transformation of the bundle of projection rays, as a result of which the vertical scale of the model is not equal to the horizontal scale. This discrepancy of scales is easily compensated as the work is being done. The operation of the correction mechanism consists in the following: when the arm is in a plumb position, the centers of the two joints of the mechanism (in the righthand part of Figure 1) coincide and no correction is made; when the arm is tilted (left-hand part of Figure 1) and there is an angle of inclination in the photograph, the centers of the joints do not coincide and a correction is made. The lens has been shifted by the quantity δ.