instruments that make it possible to take stereoscopic measurements from stereopairs in order to determine the dimensions, shapes, and spatial positions of the objects photographed. The principal parts of all stereophotogrammetric instruments, regardless of basic design or execution, are the plate holders (usually two), on which the photographs are positioned, a viewing system for viewing the stereoscopic model, and measuring marks on each member of the viewing system or in the space of the geometric model of the object re-created during projection of the two photographic images. During measurements, the operator carries out successive stereoscopic alignments on different image points of the object. He may fix the position of the points graphically or read the points’ coordinates from special counters in the coordinate system of the photograph or of a separate model, depending on the type of instrument used.

Stereophotogrammetric instruments are classified according to their function as general- and single-purpose types. The design of the former makes it possible to perform on a single instrument the entire range of processes necessary to obtain the geometric characteristics of the object under study. Each single-purpose instrument is designed to perform a single process. The stereocomparator is the most widely used single-purpose instrument.

General-purpose stereophotogrammetric instruments are divided into the analogue and analytical types. Analogue instruments reproduce and measure a geometric model of the object. Depending on the method used to construct the model, analogue instruments are classified as optical, mechanical, or opticomechanical.

An optical instrument has two or more projection cameras used to reconstruct from photographs bundles of projection rays with the relative spatial orientation corresponding to the position existing at the time the photographs were taken. A geometric model of the photographed object is constructed by means of the intersection of projection rays from conjugate image points on the photographs. The scale of the model is determined by the ratio of the projection base length—the distance between the nodal points of the lenses of the two projection cameras—to the original photographic base length. The stereoplanigraph is an example of stereophotogrammetric instruments of this group.

In a mechanical general-purpose stereophotogrammetric instrument. bundles of rays and the model are reconstructed by means of precision arms or rules that move in a plane or in space. Such instruments as the stereograph, the stereoprojector, and the stereoautograph use the principle of mechanical projection. In opticomechanical stereophotogrammetric instruments, bundles of projected rays are reconstructed optically, and the model is constructed by means of mechanical devices.

The bundles of projected rays reconstructed in analogue stereophotogrammetric instruments may be similar to the bundles that existed at the time the photographs were taken or to the transformed ray bundles; accordingly, the model obtained is similar to the terrain, or it may be a transformed model. Transformations of ray bundles arise in those cases when the distance from the photograph to the center of projection in the stereophotogrammetric instrument is not equal to the focal length of the camera lens used to take the photographs being processed. Thus, in stereophotogrammetric instruments with transformed bundles, it is possible to process photographs taken with a camera having any focal length.

Figure 1. Diagram of a stereophotogrammetric instrument with optical projection

The double projector illustrated in Figure 1 is the simplest optical-projection instrument. In order to set the orientation elements, two cameras (1 and 2) may be tilted at angles a and >, one camera (2) can move distances b_x, b_y, and b_z (the components of the base line), and the photographs (3 and 4) can rotate in their plane by angles κ. The rays traveling through the lenses (5 and 6) reconstruct from the photographs the projected rays that intersect within the instrument. Conjugate projected rays (two rays emanating from point M are shown in the figure) reconstruct the geometric model when they intersect; the model is measured by means of a table (7), which has a mark (8). The table moves freely in the plane of a screen (9). The mark is entered on a disk (10), with which it moves in the direction of the z axis. A pen (11), which may be used to obtain a graphic plan of the photographed object, is located on the mark (8).

Analytical general-purpose stereophotogrammetric instruments consist of a’stereocomparator, an electronic computer, and a coordinatograph; they have greater capabilities than analogue general-purpose instruments. The transfer from the coordinates of the points on the photographic image to those of the points of the object is made by means of the computer.

Stereophotogrammetric instruments may be equipped with special additional attachments in order to expand their range of application. Such attachments make it possible to prepare not only graphic maps but also orthophotomaps of any region. Research is also being conducted on the complete automation of the process of stereographic measurement.