Aerial Photo Interpretation

a method of studying terrain by examining aerial photographs of it, involving detection and identification of the objects photographed, determination of their qualitative and quantitative characteristics, and recording the results graphically (using standard symbols), numerically, and textually. Aerial photo interpretation has certain features typical of terrain study in general, as well as certain differences, determined by the nature of the fields (practical and scientific) in which it is used (in combination with other methods of research).

In order to obtain aerial photographs with the best informational potential for a given type of interpretation it is important to take account of natural conditions (shape of the landscape, illumination of the terrain), size and reflective capacity of the objects, selection of the scale, technical equipment (type of aerial film and camera), and operating conditions (flight photography and laboratory work).

The effectiveness of the interpretation—that is, of discovering the information contained in the aerial photographs—is a function of the characteristics of the objects studied and the nature of their transmission (by interpretative features), the refinement of the method of work, the available instruments, and the capabilities of the interpreters. The series of giveaway factors includes direct, indirect, and frequently composite indicator-features. Direct criteria include dimensions, shape, actual and incident shadows (sometimes considered indirect), and photo tone or color; a composite criterion, the design or structure of the picture. Indirect indicators are those that indicate the presence or nature of an object that is not itself evident in the photograph because of filming conditions or terrain (for example, vegetation and microrelief are indirect indicators of soddy soils).

In the matter of methodology, interpretation ordinarily combines field work and laboratory work, the scope and order of which depend on the purpose and the amount of terrain studied. Field interpretation involves either complete or selective examination of the area and determination of the necessary information by direct study of the objects to be interpreted; aerial observation is a field method used for inaccessible territory. Laboratory interpretation involves analysis of objects on the basis of their interpretative features, using various instruments, cartographic reference materials, standards (obtained by field interpretation of key sectors), and the geographic interdependence of objects that have been established for the given area (the “landscape method”). Although indoor interpretation is much more economical than field work, it does not fully replace the field work, since certain data can be obtained only at the site in question.

Automation of photo interpretation is being developed in two areas: (a) selecting the aerial photos that have the necessary information and converting them in order to improve the picture of the objects under study (done by the methods of optical, photographic, and electronic filtration, holography, and laser scanning), and (b) identifying the objects by computer comparison of the coded shape and dimensions and of the density of photo tone of a given image against a standard. This can be effective only when the conditions of aerial photography and photo processing are standardized. For this reason the immediate prospects of automating photo interpretation are tied to the use of so-called multichannel aerial photography, which makes it possible to obtain synchronous images of the terrain in different bands of the spectrum.

The apparatus used in interpretation includes magnifying instruments (magnifying glasses and optical projectors), measuring instruments (parallax rule and microphotometers), and stereoscopic instruments (portable field and pocket stereoscopes, stereoscopic glasses, and table-model stereoscopes for laboratory use, sometimes with binocular and measuring devices—for example, the stereometer). The topographic plotter is a stationary instrument designed especially for purposes of photo interpretation. Aerial photographs are also interpreted on general-purpose stereophotogrammetric instruments in drawing master maps. Depending on the task, interpretation may be done with negatives of the photos or with the prints (on photographic paper, glass, or positive film) on photographic sketches (assembled along the flight line or by areas) and on accurate photographic maps. Interpretation is done in transmitted or reflected light; the results are outlined or engraved (in one or more colors) on the photographs themselves or on clear plastic sheets that are laid over them.

There are special occupational requirements for photo interpreters regarding perception of brightness and color contrast, stereoscopic vision, and the ability effectively to recognize and analyze objects in aerial photos by their specific images. Along with these qualities, photo interpreters should know the natural and economic conditions of the territory itself and should have knowledge of its photographic conditions.

A distinction is made between general geographic interpretation and sectorial work. The former category includes topographical and landscape interpretation, the latter includes all other kinds of interpretation. Topographical interpretation has the greatest application and universality. Its objects are the hydrographic network, vegetation, ground, arable land, relief forms, glacial formations, populated points, buildings and structures, roads, objects on the terrain, geodesic points, and boundaries. Landscape interpretation results in regional or typological regionalization of the terrain.

Sectorial interpretation is used in geological interpretation (in drawing geological area maps, mineral prospecting, and hydrogeological and geological engineering work), marsh interpretation (in exploring peat deposits), forest interpretation (when inventorying and organizing forests and in forestry studies), agricultural interpretation (in drawing up land management plans and in keeping track of lands and the condition of planted fields), soil interpretation (when mapping and studying soil erosion), geobotanical interpretation (when studying the distribution of plant communities—mainly in steppes and deserts—and for indication purposes), hydrographic interpretation (in investigating inland waters and the surfaces of water basins and in studying the seas in reference to the nature of sea currents, sea ice, and the floor of shallow waters), geocryological interpretation (when studying frozen forms and permafrost phenomena), and glaciological interpretation (when studying glaciers and the formations that accompany them). Photo interpretation is also used for meteorological purposes (observation of clouds, snow cover, and so forth), in searching for commercially useful animals (especially seals and fish), in archaeology, in socioeconomic research (for example, in monitoring transport traffic), and in military affairs (when processing materials from aerial photographic reconnaissance). In many cases photo interpretation is multifaceted (for example, when used for land reclamation work).

A number of practical and scientific fields make use of interpretation of photographs taken from manned spacecraft, orbiting stations, and artificial earth satellites. In this last case reception is completely automated; either the photographs are delivered to earth in containers or the image is transmitted by television. Pictures from space make it possible to do direct interpretation of global and regional objectives and to interpret the dynamics of natural processes and the manifestations of economic activity for large areas over a short span of time. The interpretation of photos obtained from conventional altitudes and from space by means of both photographic and different kinds of photoelectronic exposure was begun in the 1960’s.