History of Surveying from Air Photographs (Especially for GATE-Geospatial 2022)

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The first ′ survey from air photographs was plotted by Laussedat in France in 1858, the photographs being taken from captive balloons or kites. Later, in 1881, Woodbury in England tried plotting from panoramic balloon photographs. In 1893, Adams in the United States patented the principle of the photographic intersection from balloon photographs, which has been the basis of several varieties of the “radial-line” method.

  • Even in those days the importance of stabilization of the aircraft, and of knowing its exact position relative to the ground, led to such experiments as those of Stolze who, in 1881, not only used a ground mark two hundred metres square on level ground to establish tilt and height, but also made a proposal for gyrostatic stabilization. Schniffer, in 1892, used wires hanging down to establish the plumb-point of the photograph.
  • Mention has been made of the small amount of progress in air survey until the beginning of the War, but it was soon extensively employed by both sides as being the only method available. In France, verticals were chiefly used, extensive ground control is available from the existing surveys. Some two thousand square miles of broken country were surveyed in great detail in Palestine on a minimum of ground control. The assumption, “states Captain H. Hamshaw Thomas, was made that ′ planes could be flown level for a certain distance at a uniform height and with wings level.” This assumption, vitally important for simple plotting from verticals, did not enable precise results to be obtained at this stage. The impossibility of measuring the height of aircraft or tilt accurately necessitated extensive ground control for good results.
  • The two types of air photograph from which surveys are plotted are illustrated below.
    • The distortion of an oblique photograph is obvious and is like the effect obtained by the amateur photographer who takes a photograph of a person in a recumbent position with his feet pointing towards the camera.
    • For large and medium scales, surveys are usually plotted from vertical photographs, while for very small scales the oblique is sometimes found to be preferable. Oblique photographs are generally “high oblique,” i.e.. . , those in which the horizon appears near one edge of the photograph. “Low obliques,” where the horizon is not seen, are used for pictorial purposes rather than for surveying. As an alternative to the oblique, in some cases, the multi-lens camera, which takes a group of photographs at one exposure, has been developed.
  • In the autumn of 1920 experiments on flying for the survey were undertaken by Professor B. Melvill Jones and Mr J. C. Griffiths of Cambridge University. A grant was made by the Department of Scientific and Industrial Research and assistance was given by the Royal Air Force. The results were most valuable in pointing out the directions in which improvements could be made because air survey was still very much “under suspicion” at this time, and the conclusions reached expressed clearly many of the limitations: then existing. The present scope of air surveys shows the great advances made since 1925. Some of the conclusions reached at that time were as follows:
  • “It is not, for instance, always possible to obtain by it the high degree of accuracy which almost automatically is realized in the majority of ground surveys. This is particularly the case in those types of aerial surveying which lend themselves to rapid and economical working; that is to say, in just those cases where the economic factor is likely to be strongly in favour of the aerial method … Stress laid upon geometric accuracy will generally, but not always, react against the aerial method.”
Ground Area Covered by Photographs
  • Most of these objections have now been eliminated by improvements in photographic apparatus and materials and in plotting.
  • The experiments showed that only specialist pilots can obtain the best results from vertical photographs. It was found that a first-class survey pilot can, in general, keep the tilt of the aircraft to less than 2°, the length of the straight strip being about ten miles, ground control being at this spacing. Maps of indifferent quality could be obtained from such photographs, provided that the differences of local height did not exceed ten per cent of the flying height. Although the recording apparatus has improved, it is only by use of automatic control that these flying conditions can be, improved.
  • In the case of levels, however, the position was different. “It is not possible to determine the absolute height of any point with useful accuracy.” Much subsequent research work has been directed towards improvement of contouring and determination of levels.
  • Experiments were also made in high oblique photography. This was found to be “suitable to any kind of country, however hilly, and is capable of favourable conditions of giving some indication of absolute height.” It was intended for small-scale mapping, the suggested method is long, straight, parallel strips up to ninety miles (the photographs at each point being two obliques and one vertical) , with ground control fifty miles apart. Errors in the distance were not greater than 11 per cent, while ground heights could be determined to within about one hundred feet. High obliques have been used extensively for small-scale planimetry and particularly in Canada, which has been well to the fore in developing air survey, and as early as 1923 the Canadian Air Board reported that much progress had been made.
  • Meanwhile, in 1921, the Air Survey Committee was instituted by the War Office. It had as its main objects the development of technique and apparatus for the production of (i) a line map at a scale of six inches to one mile, and (ii) a contoured map at three inches to one mile. These were to be comparable in accuracy with maps prepared from ground surveys.
  • Jones and Griffiths observe, “While there seems to be very little scope for the aerial survey in England, it may be a means of facilitating the progress of civilization in many lands overseas.”
  • Colonel Sir Charles Close remarked in 1924 that the position was that air survey was indispensable in war, but its uses were limited in peacetime it had possibilities of immediate application to surveys of deltas, estuaries and creeks (e. g. to Irrawaddy and Nile deltas) and also for making surveys of native towns. He concluded, “The position may change, but at present, it must be considered as an auxiliary method only, but one of particular importance in an area difficult of access to the surveyor.” Subsequent improvements and requirements have shown the adaptability of the air photographs for all types of survey, including large-scale surveys in Britain.
  • The devastated areas in France were re-surveyed by the Compagnie Aerienne Frangaise, using identifiable points of the original detail to establish the scale of the vertical photographs.
  • An indication of the commercial application of air survey for new mapping was given in 1924 from vertical photographs by the forestry survey of the Irrawaddy Delta. In this survey, one thousand square miles were photographed from a height of just over 9,000 feet, at a scale of 14 inches to one mile. Of this, waterways and unclassed forests formed three hundred and tiny square miles, and owing to the swampy land it would have been very difficult and tedious to complete the work by ground methods.

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