Aerotriangulation

a method of determining the coordinates of points of a terrain on the basis of aerial photographs. The purpose of aerotriangulation is to make a geodetic network more dense in order to provide photographs with the control points that are necessary for the compilation of topographic maps and for the solution of various engineering problems. Aerotriangulation may be three-dimensional or planimetric, depending on whether all three coordinates of a point or just the two coordinates that characterize the location of a point in the horizontal plane are determined.

Three-dimensional aerotriangulation requires that a general model of the terrain imaged in particular photographs be constructed and then oriented with respect to a geodetic coordinate system. The three-dimensional problem may be solved by means of the absolute orientation of the photographs, that is, by arranging the photographs in such a way that corresponding projection rays intersect and the coordinates of ground control points equal their specified values. This technique is known as the absolute-orientation method. A general model may also be obtained by constructing partial models on the basis of individual stereopairs of photographs and compiling the models by means of conjugate points. The partial models may be independent or partially dependent. In the analytical solution of the problems of three-dimensional aerotriangulation, the coordinates of points in a photograph are measured with a monocular comparator or a stereocomparator, and the coordinates of points of the terrain are computed. The absolute-orientation method is the most rigorous and exact technique used in aerotriangulation. It is based on the simultaneous adjustment of photogrammetric and geodetic measurements and of the readings of appropriate instruments aboard the aircraft from which the photographs are taken.

Photogrammetric instruments, such as stereographs, stereo-projectors, and plotters, make it possible to construct independent or partially dependent models. Such instruments are used for three-dimensional aerotriangulation by a technique similar to the absolute-orientation method (seeAERIAL PHOTOGRAPHIC SURVEY).

Planimetric aerotriangulation is based on a property of photographs with small angles of tilt, whereby central angles whose vertices lie at or near the principal point in a photograph are virtually equal to the corresponding horizontal angles on the terrain. Planimetric aerotriangulation may be performed analytically by measuring the central angles or the coordinates of points in photographs. It may also be carried out graphically by means of radial line plots, in which the angles in the photographs are drawn.

Aerotriangulation may be carried out either by using a single strip or by using several strips and a digital computer. The method based on several strips and a computer is more efficient, since it requires less field work in the preparation of the photographs than does the single-strip technique.