Photoelectric Aerial Survey

the surveying of terrain from the air or from space with scanning equipment that makes it possible to receive electromagnetic waves radiated or reflected by objects, to amplify the waves, and to convert the waves by means of an image converter into a visible image, which is reproduced on photographic film that is moving at the same speed as the equipment carrier. During a photoelectric aerial survey, a sequence of images is constructed by scanning, which is accomplished in the transverse direction by the operation of the scanning device and in the longitudinal direction by the motion of the carrier. Such a survey may be performed either in the visible region of the spectrum or outside the visible region.

The most important types of photoelectric aerial surveying used in practice are thermal infrared (IR) and radar surveying (seeAERIAL METHODS OF EARTH STUDY). As a rule, each of the two types requires its own conditions and surveying operations. Photoelectric aerial photographs resemble conventional aerial photographs in the overall appearance of images of terrain. However, rather than reproducing the external appearance of objects on the ground, photoelectric aerial photographs reproduce the thermal properties of the objects or the nature of radio-wave reflection from the objects. Hence, such photographs may be used as a source of additional information. The interpretation of photoelectric aerial photographs is essentially the same as that of conventional aerial photographs. However, a photoelectric image contains less detail, and a larger number of natural and technical factors that predetermine the transmission characteristics of various objects must be taken into account.

Thermal IR surveying is a form of passive—that is, non-radiating—photoelectric aerial surveying and is designed to detect an object’s own thermal radiation in the wavelength range from 1.2 to 25 micrometers (µm). Of the available atmospheric transmission windows in this range for thermal radiation, the region from 3.4 to 4.2 µm is used to detect the radiation from very hot objects, and the region from 8 to 12 µm is employed for objects that are not very hot. During a thermal IR aerial survey, scanning is performed perpendicular to the line of flight by means of optical equipment that provides a wide viewing angle, that is, an angle of the order of 60°. Present-day instruments for such surveying, which are known as airborne IR image converters, can provide aerial photographs on a wide variety of scales. In the photographs, terrain features with a size of 1/1,000 of the altitude at which a photograph is taken and temperature differences of 0.5–1°C can be resolved.

Inasmuch as IR contrasts at the earth’s surface vary considerably—that is, from season to season and during the day, depending on exposure relative to the sun, on differences in the thermal inertia of objects and in the operation of anthropogenic heat sources, and on meteorological conditions, especially cloud cover—it is advisable to make repeated thermal IR aerial surveys, including at night, of the same area in order to reveal the properties of the objects being studied. Thus, the great variability of recorded quantities, which poses considerable difficulty in the selection of survey parameters, offers additional possibilities for the reproduction of objects in aerial photographs. Repeated surveys are effective in the mapping of volcanic activity—that is, for mapping anomalous temperature zones, lava flows, and hot gases and water—and the mapping of frozen-ground phenomena, in identifying patches of wet ground, in studies of the temperature conditions and pollution of reservoirs and the nature of sea ice, in detecting streams covered by vegetation, and in mapping the sites of underground and surface fires, such as forest fires and fires in spoil banks. They are also used in the inspection of power systems and drainage structures and in the periodic monitoring of sown lands.

Radar aerial surveying is a form of active photographic aerial surveying and is designed to detect electromagnetic waves in the radio-frequency range (that is, waves with wavelengths of several millimeters to several meters) that are reflected from objects on the ground. A radar system aboard the carrier serves as the radiation source and detector. In cartography, side-looking airborne radar operating at wavelengths of 1–3 cm is the most widely used radar system. Scanning is carried out by means of special antenna equipment and makes it possible to obtain an image of terrain in the form of two wide strips parallel to the line of flight. The scales used most often for radar aerial photographs range from 1:60,000 to 1:400,000. The highest resolution for terrain features is 3–5 m. The reproduction of objects on the ground in the photographs depends on the intensity of the radio-wave reflection from the objects; the intensity, in turn, depends on the properties and shapes of the objects as well as on the steepness of the topography and the direction in which the topography slopes. By taking these relationships into account and varying the main radar parameters —that is, the wavelength, pulse rate, and pulse shape—the required separation of the images of the objects being studied may be achieved in the photographs. Radar aerial surveying may be conducted regardless of the time of day and atmospheric conditions; that is, it is an all-weather technique.

Because radio waves can reach a depth of tens of centimeters beneath the earth’s surface, geological prospecting and the study of ice are the main fields in which radar aerial surveying is used. It is especially significant that radar aerial surveying, in comparison with conventional photographic surveying, provides for substantially better interpretability of, for example, the following: tectonic faults; the nature of rocks covered by vegetation, snow, or surface detritus; the mechanical composition, especially the particle sizes, of surface detritus and the presence of metallic constituents; the structure of ice formations; and cracks and meltwater channels in ice. Objects on the ground that are immersed in dark shadows are reproduced more clearly in radar photographs than in conventional photographs. Since stereoscopic models of terrain may be constructed from radar photographs and heights may be determined with an accuracy of up to 15 m, radar photographs are used when certain inaccessible regions, such as polar wastes and equatorial jungles constantly obscured by clouds, are studied for the purpose of making survey-type topographic maps.