The Stereoautograph is a complexe opto-mechanical measurement

Measurement

Measurement is the process or the result of determining the ratio of a physical quantity, such as a length, time, temperature etc., to a unit of measurement, such as the metre, second or degree Celsius...

 instrument for the evaluation of analog or digital photogram

Photogram

A photogram is a photographic image made without a camera by placing objects directly onto the surface of a photo-sensitive material such as photographic paper and then exposing it to light. The result is a negative shadow image varying in tone, depending on the transparency of the objects used...

s. It is based on the stereoscopy

Stereoscopy

Stereoscopy refers to a technique for creating or enhancing the illusion of depth in an image by presenting two offset images separately to the left and right eye of the viewer. Both of these 2-D offset images are then combined in the brain to give the perception of 3-D depth...

 effect by using two aero photos

Aerial photography

Aerial photography is the taking of photographs of the ground from an elevated position. The term usually refers to images in which the camera is not supported by a ground-based structure. Cameras may be hand held or mounted, and photographs may be taken by a photographer, triggered remotely or...

 or two photograms of the topography or of buildings from different standpoints.

The photograms or photographic plate

Photographic plate

Photographic plates preceded photographic film as a means of photography. A light-sensitive emulsion of silver salts was applied to a glass plate. This form of photographic material largely faded from the consumer market in the early years of the 20th century, as more convenient and less fragile...

s are oriented by measured passpoints in the field or on the building. This procedure can be carried out digitally (by methods of triangulation and projective geometry

Projective geometry

In mathematics, projective geometry is the study of geometric properties that are invariant under projective transformations. This means that, compared to elementary geometry, projective geometry has a different setting, projective space, and a selective set of basic geometric concepts...

 or iteratively (repeated angle corrections by congruent rays). The accuracy of modern autographs is about 0.001 mm.

Well known are the instruments of the companies Wild Heerbrugg

Wild Heerbrugg

The Wild company was founded in 1921 in Switzerland. The company manufactured different optical instruments, such as Surveying instruments, microscopes and instruments for photogrammetry among others...

 (Leica), e.g. analog A7, B8 of the 1980s and the digital autographs beginning in the 1990s, or special instruments of Zeiss and Contraves.

Sources

• Gilbert Willy
• Military Topography and Photography By Floyd D. Carlock, U.S. Army, 1916, p.104 ff, with photos (Available online at Google Books)