Development to Aerial Photography as a Tool in Modern Geography: Camera Lens Design, Photographic Material, Photogrammetric Plotting and Dual Aerial Photography in Geography (Especially for GATE-Geospatial 2022)

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The ‘Quantitative Revolution’ in the mid-1960s has brought a ‘New Geography’ which takes the search for models and theories as a recurrent theme of study. This trend towards a nomothetic approach has enhanced the importance of aerial photography as a tool in collecting accurate and up-to-date data for testing geographical hypotheses and models.

Difference between Satellite Imagery & Aerial Photography

The term aerial photography by itself is usually applied in a rather restricted sense to mean the taking of photographs from an airborne platform. In this book, its use is much wider to cover the entire field of the study of aerial photographs and is, therefore, more or less synonymous with the term aerial surveying, or air survey, which embraces both the methods of photogrammetry and photo-interpretation.

The development of aerial photography has taken place comparatively recently and has been heavily dependent on advances in optics and aircraft technology. When the first lens was ground in 1812, the era of photography began which ultimately led to the development of better lenses such as the distortion-free Hypergon lens covering a view of 72° and the invention of the Daguerreotype camera as a practical means of photography in the 1840s. In 1858 the first aerial photographs were produced in France by Nadar (Gaspard Felix Tournachon) using the Daguerreotype camera and a balloon. From these photographs, the Frenchman Aims Laussedat, generally known as the Father of Photogrammetry, applied the method-perspective to ″ produce a map on paper. ′ Unfortunately, because the limitations of balloons or kites as camera platforms prevented the taking of enough photographs to cover all the area required, no further advances in mapping using aerial photography occurred until 1903 when the Wright brothers successfully developed the aeroplane and made flying possible. Until then, interest centred on terrestrial photogrammetry, i.e. the use of ground photographs for mapping. In 1909 the first photographs taken from an aeroplane were made. During World War-1- aerial photography rapidly-expanded as its military importance was realised, which resulted in great improvements in the techniques of photo-interpretation. As aerial photography became more generally available, the advantage of aerial photography in giving a bird ′ s-eye view, or the holistic view of our terrestrial environment became apparent. It immediately attracted the attention of earth scientists, and as early as 1920 the potentialities of aerial photography for use in archaeology, botany, geography and geology were made known. A good example of early geographical applications reflecting a typical ′ regional approach ′ was the work by Lee, who used aerial photographs to describe the ′ face of the earth ′ .

Subsequent developments in aerial photography have always been aiming at improving the accuracy of the quantitative (or metric) as well as the qualitative (or descriptive) data extracted from aerial photographs. These have been achieved in three directions:

  • Camera lens design
  • Photographic materials
  • Photogrammetric plotting machines

Camera Lens Design

The angular coverage of the lens was increased, for example, the Topogon lens manufactured by Zeiss of Germany in 1933, covering an angle of 90°, and the Metrogon lens by Bausch and Lomb in the United States after World War II. This made possible wide-angle photography, which gives rise to a more favourable base/height ratio (i.e.. the ratio of the separation between two successive exposure stations from the camera to the object) for better heightening accuracy than the narrow-angle (45°) photography. Another advantage is the considerable saving in the number of control points required in photogrammetric plotting since a smaller number of photographs is required to cover a larger area. Today, the use of wide-angle photography is generally accepted as the standard practice in photogrammetry. In 1959 the Swiss firm of Wild manufactured the Super-Aviogon lens which covers a view of 120°, thus increasing even further the angular coverage of the camera (i.e. super-wide-angle camera) .

Meanwhile, the metric quality or the precision of the lens has also been considerably improved through more accurate computation made possible by the electronic computer. Modern camera lenses are claimed to be distortion-free; the Pleogon lens of Zeiss (Oberkochen, West Germany) and Aviogon lens of Wild, for example, claimed a distortion of between ±5 and 6 pm.

Photographic Materials

The photographic process makes use of the fact that visible light (which may be considered to lie between 400 and 700nm of the electromagnetic spectrum) can react with a silver halide salt to form an invisible latent image, which, after, development by chemical means to reduce the silver halide grains to silver, results in a negative image with densities proportional to the intensities of light received. The sensitivity to light of this chemical emulsion is important. The speed of the emulsion today has been increased by over 4 million times as compared with 1839; for example, the Eastman Kodak Tri-X film possesses a speed of ASA 200 (or DIN 25°) . The ability of the emulsion to detect fine details (i.e.. the resolving power) has been greatly improved. It is also possible to produce emulsions of different spectral sensitivity (i.e.. to respond to light of different wavelengths) for different purposes of the application, hence, the three different types of sensitising known as orthochromatic, panchromatic and infra-red. Even the recording of colour is possible by combining in a film three layers of emulsions which are separately sensitive to red, green and blue light. Another great improvement, made notably by the Eastman Kodak Company, is the dimensional stability of the film base on which emulsion could be coated so that now photogrammetry can be released from using the heavy glass plate needed in the past. Such a film base, which is made of polyester material, is variously known as Mylar, Estar, Cronar, etc, but is generally called a Topographic Base.

Photogrammetric Plotting Machines

The rapid production of topographic maps from aerial photographs has been made possible by using machines which can solve problems by analytical or analogous means the geometric of aerial photographs in order to obtain correct topographic maps, e. g. the stereo comparator invented by C. Pulfrich of Zeiss in Germany in 1900 and the Stereoautograph by E. von Orel in Austria in 1908 The most important development was stereophotogrammetry which requires the use of stereo pairs of photographs for plotting in the machines (hence the name stereoplotter) . These photographs can be aerial or terrestrial, and in 1923 a universal machine called the Stereoplanigraph designed by Professor Bauersfeld was successfully produced in Zeiss, Germany, which accepted both terrestrial and aerial photographs in stereo pairs in plotting maps. This was followed by the emergence of another machine - the Multiplex by Zeiss in 1933 which had a great impact on photogrammetric mapping in the United States. Today, the variety of plotting machines is even greater, notably, the series of machines manufactured by the Swiss firm of Wild (e. g. A7, A8, A9, A10, B8, B9) . Most important is the availability of a cheaper range of stereoplotters (known as topographic plotters) which can achieve a reasonable degree of accuracy for more general applications, apart from topographic mapping using small-scale or medium-scale photography. Examples are the Wild B8 and the Kern PG2.

It is clear from the above examination of the three major lines of development that the aerial camera is, in fact, a powerful and precise surveying instrument which is capable of recording the minute detail of our terrestrial environment subject to the limitations of the scale of photography and the resolving power of the film. Thus, with the geographers՚ concern for the terrestrial environment, the use of aerial photography in geographical studies was well established in the period just before World War II and the term photo-geography was generally used to refer to the subject. Not surprisingly, interest mainly centred on regional geography in which aerial photographs were used as a kind of supplement to maps for the delineation of regions. In modern terminology, these regions are areal classes which have uniform characteristics throughout, and the whole process of regionalisation is analogous to the technique of classification which is generally employed at an early stage by other natural sciences, notably botany and zoology (where the term taxonomy is used) . Therefore, the main use of aerial photographs at such a stage is to provide a base for regional inventories. Although the inventorying is necessarily descriptive, the approach is still useful today when a reappraisal of the man-land relationship and the maintenance of the ecological balance are called for in geography. Paradoxically, there is also the need to develop further the underdeveloped areas found within developed nations such as Canada (e. g. the Northern Lands) as well as in developing nations (so-called Third World countries) . Natural resources inventorying and topographic mapping in these areas can benefit greatly from the appropriate use of aerial photographs. One of the modern approaches is to identify the repetitive patterns of similar tones and textures appearing on photographs over a broad area as photographic units (equivalent to geographic individuals) , which can be employed for evaluation purposes. The ability of aerial photographs to provide environmental data from the spectral, spatial and temporal standpoints is invaluable in a new regional approach, the i.e. regional analysis in which a higher degree of objectivity and quantification is characteristic.

The Importance of Stereo Models

One of the most valuable properties of aerial photography is its ability to give rise to a stereo model. The stereo model is necessarily a mental model obtainable in one՚s mind by viewing a stereo pair of photographs. It is made possible by the pair of human eyes which, by means of convergence (i.e.. gazing at the object) and accommodation (i.e.. focusing the eye-lens) , fuse the two images together to form a three-dimensional model. Usually, the stereo model is seen with the aid of a stereoscope to magnify and to maintain the three-dimensional impression. There are various theories explaining the formation of the stereo model, and it is obvious that some physiological and psychological factors have been involved when one perceives the three-dimensional model. Raasveldt attempted a reconstruction of the stereo mode in space by employing a geometrical theory of projections. Modern research has shown that the image formed by the eye՚s optical system is sensed in the retina by a photochemical process as the first section of the visual pathway. This information is transmitted along the visual pathway until it reaches the occipital cortex the brain which permits the visual perception of the three-dimensional model. It should be noted that experience plays an important part in aiding stereoscopic or binocular vision. Thus, the stereo model is a spatial analogue model giving a small-scale representation of the reality. Geometrically, the stereo model as seen by an - observer is incorrect because the photographs are usually placed on the flat table underneath the stereo-scope without taking into account any tilts that may have been present at the time of photography, and because the photographs are not generally viewed at a distance equal to the principal distance of the camera lens. An affine-deformed stereo model (i.e.. a linear scale deformation of the model in the vertical direction) results, giving rise to different scales in planimetry and height. Another defect of such a stereo model is that it is highly subjective. On the other hand, it is possible to rectify all these defects by using more sophisticated machines, such as the stereo plotter which can take out any tilts present and the densitometer which can measure the tonal values from one part of the photograph to the next, thus making possible more objective interpretation.

The Dual Functions of Aerial Photography in Geography

This brief survey has revealed two major functions of aerial photography in geography today, namely, as a data-collecting tool and as an analytical tool. It is particularly not worthy that aerial photography, by virtue of its special vantage-point, is an efficient surveying tool for inventorying and mapping. In other words, it can collect data from the spatial, temporal and spectral standpoints. On the other hand, the stereo model as generated from special stereo-photography can be treated as an analogue model of the terrestrial environment, which can be continuously simplified, by changing into successively smaller scales, to give rise to some conceptual models in geography, thus meeting the need for a more theoretical application of the ‘New Geography’ . As such, aerial photography is acting as an analytical tool in geography. 11 At the same time, aerial photography has been undergoing tremendous improvements, especially towards refinements in the techniques of photogrammetry and a more objective approach in photo-interpretation. All these have combined to enhance the value of aerial photography as a tool in modern geography.

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