Air Survey Methods and Plotting: Review of Literature, Techniques and State of Art (Especially for GATE-Geospatial 2022)

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Before tracing the development of air survey technique, it is desirable to separate the various methods by which maps and plans can be produced. In each case, attempts have been made to satisfy requirements of accuracy and scale for definite types of country.

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The conclusions reached by Jones and Griffiths in 1925 were that “possible methods of aerial surveying can be sharply divided into two groups, one of which will give accurate results, but is not very economical, whilst the other may be very economical but is as yet incapable of giving very high accuracy. The methods of the first group which have been intensively studied in Germany will be of value in connection with surveys in which the factors of accuracy and detail are of great importance, as compared with the factor of the economy.”

In the 1935 report of the Air Survey Committee, it is stated that “almost inevitably … photographic methods of surveying imply precise measuring apparatus and/or specially designed optical methods of enlargement. Consequently, the development of aerial surveying consists largely in the invention and design of apparatus which will measure the much-reduced photographic images of the landscape in the most convenient, the most rapid, and the most accurate manner.”

In almost all cases, even if the plotting is not based on a stereoscopic method, stereoscopic interpretation of the photographs is employed.

Main Methods of Air Survey

The main methods of air survey may be grouped as follows:

  1. Graphical or semi-graphical methods of plotting from vertical photographs for medium and fairly large scales.
  2. Very accurate methods on large scales, contours and plan being produced by elaborate machines.
  3. Mechanical methods of plotting plan and contour for medium scales with minimum ground control.
  4. Small-scale surveys by oblique photography, from photographs taken with a multi-lens camera, or with an ultra-wide-angle lens.

Simple Methods of Plotting from Vertical Photographs

Plotting machines are very expensive and can be employed only by large organizations. Even then it is considered by some that such instruments may prove a “bottleneck” in the work and the War Office have therefore directed much of their attention towards plotting by simple methods from photographs taken as vertically as possible in strips, with about sixty per cent longitudinal overlap between photographs to ensure stereoscopic vision over the entire area.

It was shown that from a point on the ground vertically below the lens, known as the ground plumb point, the angle between any two lines was equal to that subtended at the point vertically above the lens, known as the photo plumb point. Great difficulties were experienced, due to unknown tilts of the aircraft at an exposure which made the plumb point indeterminate unless extensive ground control was available; and to distortions caused by varying heights of the points. Instruments such as the McLeod Tilt Finder was designed to cope with this problem but were not used extensively except in those large-scale maps where the photographs were rectified with respect to ground control before plotting. Efforts were made to improve flying conditions so that a graphical routine having known limitations could be evolved.

McKinley describes this “radial line” method used in the United States. Although this technique is the basis of the Arundel Method, much of the credit for the latter must go to Hotine, who was at the time Research Officer to the Air Survey Committee. He worked out a definite procedure for graphical plotting, aided by stereoscopic examination. At first tilt distortions limited application to areas where the height did not vary by more than ten per cent of the flying height, a ground control point is required every five to ten miles for the plan and five or six points per overlap for levels. Recent improvements of photographic materials and the use of the automatic pilot have increased the scope of this method. Contouring by simple methods still presents many difficulties, and without extensive ground control, or ground levelling, it seems almost inevitable that a plotting machine will be required.

A modified Arundel Method has been adopted by the Ordnance Survey for the 1/2,500 revision and overhaul, which does not include levels. The photographs are corrected for tilt by rectification with respect to at least four known points per overlap, while the Thompson Comparator has been designed to eliminate some stages of the graphical plotting. An earlier instrument still sometimes used for revision is the Barr and Stroud Epidiascope where the photograph is projected onto a screen in coincidence with identifiable points on the old plan so that the amendments can be traced direct.

In this, as in all large-scale surveys, groundwork and measurement are necessary to fill in the data which cannot possibly be supplied by the air photograph.

Even when using simple methods, vertical photography becomes un-economic if the scale is much less than 1/30,000 unless one of the new ultra-wide-angle lenses is employed. Limitations of flying height and focal length, together with the time taken in plotting and reduction, encourage the use of multi-lens or oblique photographs.

Accurate Large-Scale Surveys

Most of the work in this field has been done on the Continent. Here in many cases, an adequate ground control exists and there are few large-scale plans published. The photographs are taken in stereoscopic pairs, the most commonly used method of plotting being that of stereoscopic fusion.

Although fairly large-scale plans can be produced by simple methods, any attempt to produce accurate levels necessitates the employment of special instruments. The early machines used in this work were modifications of those used formerly for plotting from ground survey photographs. Apart from special instruments for rectifying photographs in relation to ground control, few special developments took place until the appearance of the Hugershoff Autocartograph in 1920. This machine had appeared somewhat earlier in a cruder form, while a further improved model appeared in 1925. It was rather cumbersome and elaborate and consisted of three principal parts.

  1. The stereoscopic observing system,
  2. the surveying system, formed by two theodolites coupled by levers for computing,
  3. a plotting system. The procedure was to set a pair of photographs by stereoscopic observation into the correct relationship as at exposure, with reference to the horizontal plane and four ground control points. The setting of the stereoscopic floating mark to the ground at various points operated the plotting mechanism and automatically produced a contoured plan.

Review of Literature of Air Photographs

In 1923 it was estimated that for one pair of photographs two days were required for computation and preliminary work, and one day for plotting on the machine. The reference plane was horizontal. Hotine points out that in using the apparatus a tedious computation is required to affect a resection in space in order to fix the three-dimensional coordinates of the two air stations forming the end of the line.

In 1924, Barr and Stroud produced a photogrammetric plotter, which was later abandoned in favour of an instrument employing the Fourcade principle.

Meanwhile, the Zeiss Stereoplanigraph and the Wild Autograph were being developed on similar lines, while the Hugershoff Autocartograph was superseded by the Hugershoff-Heyde Aero-cartograph.

In 1926 Fourcade introduced his “correspondence” theory, and thence evolved the Fourcade Stereogoniometer (described later) in which, for the first time, the setting of the photographs was related to the air-base. This omission in earlier instruments caused great trouble because the air-base is rarely horizontal, the aircraft being unable to fly along a horizontal plane. Fourcade showed that if five points can be accurately identified on each photograph of a pair, the photographs of the pair can be set in their correct relationship to each other without reference to ground control. One great advantage of setting with relation to the air-base is that orientation is greatly accelerated. A plotter for use with the Fourcade instrument has been devised by Captain E. H. Thompson, R. E. , lately and Research Officer to the Air Survey Committee and now Air Survey Officer to the Ordnance Survey. In 1932 Professor Hugershoff became associated with the firm of Zeiss and the Hugershoff-Heyde Aero cartograph has been abandoned in favour of the Stereoplanigraph which has been much improved in design. The Wild Autograph and other instruments have also been much improved in design and facility of operation. (The Poivilliers-S. O. M. Stereotopograph is similar in principle and achievement to the Wild and Zeiss Instruments)

In France, an air survey has been extensively employed, first for re-mapping devastated areas and subsequently for town planning purposes. The method, which is similar to that which has been used extensively in this country, is to take photographs as nearly vertical as possible and rectify photographically with respect to ground control.

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