PHOTOGRAMMETRY (Fr., Photogrammetrie, Metrophotographie ; Ger., Photogrammetrie, Messbildverfahrung) The science of measuring and surveying by the aid of photography. The camera is exten sively used in the preparation of maps and for other topographical purposes. With a high class lens free from distortion, a rigid camera and stand, and a fixed vertical position of the plate with regard to the lens, the size and arrange ment of objects in the negative will bear a con stant ratio to those of the original subject. If two or more photographs are taken, for in stance, of any prominent building or natural feature from different standpoints, noticing carefully the part of the compass to which the camera is directed in each case, it is possible by means of the various points appearing in the resulting negatives or prints, and by a series of triangulations, to draw an accurate dimensioned plan. The camera for photo grammetric purposes is generally provided with levelling screws and a graduated circular scale at the base, while four fixed points are arranged at the back to register on each plate exposed the position of the horizontal line and of the lens axis. It is usual, also, for a magnetic compass and a theodolite to be attached to the camera.
The subject is too large to treat otherwise than briefly, but a few of the simpler formulae used in photogrammetry may be quoted. It is assumed that the lens is at its principal focal distance from the plate and that the camera back is strictly vertical. Let 0 = height of object, I = height of image in negative or print, F = focal length of lens, and D = distance of object from lens, measured from nodal point. Then, to find D.
height of object, 0 = I The width may be found in the same way, provided the plane of the object is known to be parallel to the focusing screen. If the size of the object is known the distance may be calculated, D = 0 distance and size of an inaccessible object may be found by taking two photographs at a measured distance apart, but each in line with the object—i.e. the latter should be central in both negatives. The distance apart may be measured either from the nodal point -of the lens, or more conveniently from the focusing screen. Let S = the distance between the two standpoints, and = the heights of the two images in the photographs, then 0 = S x x I2 while D= — or OF • F x (I, —