PHOTOGRAPHED FROM A HEIGHT OF 15,000 FEET. This picture of Durban Harbour, South Africa, was taken during an air survey of the district for mapping purposes. Air photographs are widely used to indicate the possibilities of navigation and to show early signs of erosion.
FEW people realize that only a small area of the world has been accurately surveyed and mapped. The report of the Air Survey Committee, 1935, shows that the areas mapped on a scale of 1:25,000 or larger are restricted to the British Isles, Germany, Denmark, Belgium, the Netherlands, parts of France, Italy and Central Europe, and a few small areas elsewhere in the world. Areas mapped on a scale of 1:100,000 or larger comprise a considerable part of Europe, all Japan, parts of the United States and India, and isolated regions in South America, Africa, Asia and Australasia.
The greater part of the world is inadequately surveyed, as the maps are on scales smaller than 1:100,000. As most of the topographical information is based on local reconnaissance surveys or on reports of travellers, the accuracy of many maps is doubtful. In the British Empire about four-fifths of the total area is inadequately mapped. The work of the pioneers of air survey has brought the science to the stage where its benefits are available not only to Governments, public bodies and large mining, engineering, railway, electrical and other undertakings, but also to the individual farmer, prospector and citizen at a cost within his means.
Since the war of 1914-18 and the time of the early air surveys made by military machines the science has become so specialized that most Governments contract with selected companies. British air survey companies using specially designed aircraft have sent survey units to various parts of the world. Aircraft, cameras, films, mobile laboratories, personnel and transport, sometimes directed by radio from a base, operate in virtually unknown territory, or, perhaps, survey and map a great city so that it can be replanned for the benefit of its inhabitants.
There are, in the British Empire, various important air survey companies, four of which are closely associated. These four are the Aircraft Operating Company, Ltd., the Aircraft Operating Company of Africa (Proprietary), Ltd., H. Hemming and Partners, Ltd., and Aerofilms, Ltd. Another company, Air Survey Company, Ltd., has a subsidiary, Indian Air Survey and Transport, Ltd.
These companies have made air surveys in widely scattered parts of the world, not only in the British Empire but also in South America, the Middle East and parts of Africa outside the British Empire.
Southern Africa affords a particularly interesting example of the extension from the State to the individual of the benefits of air survey. The science has progressed from the first extensive survey in Northern Rhodesia in 1926 to the stage where a farmer or prospector can not only have land photographed on a scale of 1:10,000, but can also have an interpretation of the photographs should he desire it. The original survey, made by the Aircraft Operating Company, Ltd., was over about 12,000 square miles on behalf of the Rhodesian-Congo Border Concession, Ltd. Oblique photographs were taken and the results were plotted on a scale of 1:250,000. The method used was a modification of the Canadian perspective grid method produced by the late Major C. K. Cochran-Patrick, D.S.O., M.C. So satisfactory were the results that the Government of Northern Rhodesia contracted with the company for a similar survey of 65,000 square miles. This extensive work was completed in eighteen months. The photographs were taken in fifty flying hours spread over a period of three months; the preparation of the maps required fifteen months. The total cost was £1 a square mile.
The completion of this survey marked a notable stage in air photography. It was an object lesson in the efficient use made by ail concerned of the equipment at their disposal, and it provided data upon which further surveys could be based. Moreover, it showed various authorities the possibilities of using air photography to serve a variety of purposes. The country was so difficult of access that a ground survey would have occupied many years and the cost would have been considerable. An official of the Colonial Office, London, who visited Northern Rhodesia, reported that, in his opinion, air photography combined with air reconnaissance and ground control would indicate geological formation in sufficient detail to serve as a basis for geological maps of the areas most worthy of intensive inspection.
Supplementing Ground Surveys
In addition, he considered that a combination of air photography and air reconnaissance would indicate localities in which mineralization was likely to have occurred, zones favourable for farming and zones in which forest ought to be preserved. The method would indicate also the correlation between the tsetse fly and climate, geology and vegetation. He further considered that the method would show alternative alinements for roads and railways to save waste in construction and maintenance and to enable productive areas to be tapped. Air photography is not a substitute for ground surveys. The pilot and photographer produce a clear photograph, the ground surveyor interprets it, collects the information and produces it in map form. The air surveyor does not attempt to do the work of the geologist or of the forestry expert. This is done on the ground with the assistance of the air photographs.
Air photographs provide valuable evidence to authorities in an ever-increasing number of professions. Sometimes geologists, forestry officials and other specialists accompany pilot and photographer in the survey aeroplane, the latest type of machine having accommodation for them as well as for the pilot, photographer and equipment.
DETAILS OF THE SPECIAL B.A. DOUBLE EAGLE designed for air survey work. The numbers represent the following items: 1, Elevator unit; 2, Rudder unit; 3, Accumulator; 4, Air drier; 5, Drift sight control; 6, Generator; 7, Air-pressure gauge; 8, Test cock; 9, Oil reservoir; 10, Pitch control; 11, Lateral trim lever; 12, Pitch control; 13, Course change cock; 14, Main control cock; 15, Drift sight; 16, Aileron gyro unit; 17, Azimuth control; 18, Williamson camera (cross supports not shown); 19, Photographer’s place; 20, Cable and pipe runs; 21, Gipsy Six (Series II) engine; 22, Air compressor; 23, Air-expansion chamber; 24 Auxiliary fuel tank.
Such accommodation is provided in the Double Eagle, built by the British Aircraft Manufacturing Company, Ltd., for the African company of the group. This three-quarter high-wing, semi-cantilever monoplane has a wing span of 41 feet, the length being 29 feet 10 inches; it is powered by two Gipsy Six Series II engines. The Williamson Eagle camera takes vertical photographs through the floor, or oblique photographs through the removable windows. Among other items of equipment are a special A.O.C. (Aircraft Operating Company) sight, a course and distance calculator, an automatic pilot installed in three separate parts, an artificial horizon and directional gyro, a sensitive altimeter and a floor drift-sight.
A specially built twin-engined Gloucester was flown to photograph much of the large survey of Northern Rhodesia. Its revolutionary design enabled the air survey of some 65,000 square miles to be made with the use of only two landing grounds. The twin engines of the Gloucester gave a security that was appreciated by the crew flying above virtually unknown territory. Some of the surveys at this early period were made to take vertical photographs of townships for planning purposes. Another air survey provided a strip mosaic (see the chapter on “Air Photography”) of some 450 miles of the Zambezi River so that the possibilities for navigation might be investigated.
The value of air photographs is not confined to the single purpose for which they are taken. Major H. Hemming, A.F.C., F.R.Ae.S., one of the pioneers of air survey and a co-founder with Mr. Alan S. Butler, of the Aircraft Operating Company, Ltd., has stated that some air photographs taken in Canada for one purpose were finally used for a variety of purposes by various Government Departments and by private enterprise interested in the area photographed. The results, in the original purpose alone, fully justified the cost of the photographs.
Specialists, during their examination of air photographs, find much evidence which conveys nothing to the layman. A petroleum technologist in the United States examined air photographs taken for mapping purposes. The photographs disclosed certain geological characteristics which showed to his trained eye that oil might be located. A new oilfield was discovered when the site has been examined.
The Significance of Shades
Great as is the value of air photography of little known areas, it is comparable with the value in areas which are well known. Frequently evidence not visible to the eye of a person either on the ground or in the air is shown in air photographs. Provided that the detail has actinic properties (that is, is capable of producing effects on a photographic negative), the details can be recorded on a photograph although invisible to the human eye.
Panchromatic films record light values which can reveal to the trained eye detail previously unavailable by the methods used before air photography was perfected.
On one photograph a thin line was shown; this was found to be a mineral lode which had affected the soil and vegetation. Trenches could be seen which had been made by prospectors to trace the direction of the lode. If the photograph had been taken before ground prospecting had been begun, much of the field work entailed in trying to trace the direction of the lode would have been avoided.
Small differences in tone in the photographs enable the observer to identify different soils, grasses and trees (see the chapter on “Air Photography”).
In the Rhodesias, the Union of South Africa and neighbouring countries air photography is proving of inestimable value. The difficulties in the way of preliminary surveys by orthodox methods have been surmounted by air surveys, so that air photographs are regarded not as an interesting novelty but as an essential part of any development or enterprise.
As much of the country is at an elevation of several thousand feet, the aeroplane generally flies at a height of about ri,000 feet above the ground; this is the most economical height to cover about two miles of country for each photograph on a scale of about six inches to the mile. When an air photograph is required for prospecting a mining claim, the photograph shows numerous details which facilitate the owner’s work by showing him where to make his investigations on the ground. The photograph enables him to decide the best position and to discover the obstacles lying in the path of his operations and their relative positions and sizes. He is able to determine the probable source of streams or watercourses and their trend, and to locate any item of interest on his claim. By studying the photographs in a stereoscope the owner will see the ground standing out in perspective; thus he will be able to judge heights and slopes and to distinguish various growths of trees.
Farmers and owners of plantations are assisted by air photographs that enable them to mark different stages of growth, the signs of pests or crop diseases. The position of a watershed, the trend of the drainage and the effects of excess water are revealed to the property owner, and enable him to decide his plans for dealing with the question of water supply. Moreover, air photographs often reveal the cause of soil erosion, its source and direction, so that the farmer can counter the menace by contour furrowing. The classification of different soils, facilitated by air photography, affects farmer and miner. Some of the most modern cities in southern Africa are being improved by the use of air photography. The Aircraft Operating Company of Africa (Proprietary), Ltd., was formed in 1931 from the staff of the original Rhodesian expedition sent out by the Aircraft Operating Company. One of the new company’s first contracts was in connexion with the town-planning survey on various scales of Durban and its environments. Having completed this survey, the company removed to its present headquarters in Johannesburg, Transvaal. Expansion has been rapid and the company has carried out air surveys, under the management of Capt. C. R. Robbins, M.C., D.F.C., in the Union of South Africa, Swaziland, South West Africa, Northern and Southern Rhodesia, Portuguese East Africa, Kenya, Tanganyika and Uganda.
Flying Map for Imperial Airways
The variety of the surveys made by this company indicates the use made in southern Africa of air maps as the key to the further development of a vast country. Among the purposes for which surveys have been undertaken are town planning, railway surveys for new alinements, deviations and improvements to goods depots. Geological surveys were made for several Governments. Some of these surveys were for revision purposes, others were over new ground. There were prospecting contracts for preliminary investigation over thousands of square miles, in addition to more intensive surveys for prospecting or for mining.
Other contracts involved timber and forest surveys, surveys for alluvial gold or diamonds, or for industrial plants, and the preparation of a flying map for the former Imperial Airways route through Northern Rhodesia between the Southern Rhodesia and Tanganyika boundaries.
The mapping of the air route entailed the survey of 16,000 square miles of new area and the redrawing of 30,000 square miles of the area formerly surveyed for the Government of Northern Rhodesia. The map, produced in colour, was believed to be the first air map for flying produced entirely by aerial methods. It depicted, among other details, forest areas, hills, settlements, villages, railways, roads, power lines and farmed areas. The company forms the photographic and air survey reserve of the South African Air Force and has been entrusted with the making of an air survey of the Union and of neighbouring territories.
Another of the associated companies, H. Hemming and Partners, Ltd., has carried out some notable contracts in Australia and New Guinea. The company made an air survey of the areas worked by the Bulolo Gold Dredging Company. It also carried out an extensive air reconnaissance of some 22,000 square miles in Papua, New Guinea, for one of the big mining groups.
In Australia the company secured a contract with the Western Mining Corporation, and organized an air surveying expedition under the leadership of Wing Commander F. C. V. Laws, O.B.E. The expedition carried out air surveys and air transport over extensive concession areas covering some 88,000 square miles in the gold-fields of Western Australia.
INSTRUMENT INDICATIONS are photographed on to the side of each negative exposed during an air survey. The instruments include a sensitive altimeter (an instrument which counts the exposures), a clock and sometimes a spirit level (not shown in this illustration). There is also a place in which the photographer writes the name of the subject, the approximate height of the aircraft and the focal length of the lens in use. This photograph was taken over Port Elizabeth, Cape Province, South Africa.
The remoteness of some of the areas in Western Australia from centres of civilization, and the difficulties of climate, territory and distance were obstacles that were surmounted by efficient planning, equipment and operation.
The unit arrived in Australia with two D.H. Dragon aeroplanes adapted for air photography. Williamson Eagle IV cameras with special lenses were used, and the aeroplanes were fitted with radio, as wireless was used extensively in the survey. Three portable radio transmitters and receivers were taken to Australia to be fitted to motor lorries, in addition to field radio sets and complete sets of spare parts. General equipment included an auto-focusing enlarger, contact printers, a copying camera, a measuring stereoscope and survey instruments.
Further equipment, added in Australia, included six transport vehicles, with a specially designed refrigerated dark-room trailer and a second trailer carrying the drying drums and base radio. The refrigerated darkroom trailer was designed to overcome the climatic conditions, which were adverse to photography. It proved successful; the photographs were clear and the definition was excellent. Much of the success of photography in a difficult climate depends upon the selection of the film and light filters suited to local conditions. Before operations could be begun, therefore, research work was necessary to find the best method of securing the widest possible range of tone values, so that the results could be interpreted for geological information.
The headquarters were at Perth, the capital of Western Australia. A hangar for the aeroplanes was built at Kalgoorlie, the gold-mining centre on the Perth-Adelaide air and railway route (see the chapter on “Australia’s Civil Aviation”). There were also drawing offices, dark rooms and staff accommodation. Sometimes the men were photographing country 800 miles away from headquarters. Radio enabled the operations manager to keep in communication at this distance. When clouds hampered photography the aircraft were directed to another field where visibility was greater.
The company, through its associated companies in South Africa, has carried out numerous surveys, some of which were in cooperation with geophysical and diamond-drilling units operating in the Rand.
Effects of the Atmosphere
The associated company, Aerofilms, Ltd., founded by Claude Grahame-White and F. L. Wills, has been conducting air surveys in Great Britain since 1919. The contracts include surveys for town planning, parliamentary applications, public services, engineering schemes, archaeological research, estate development, railway surveys, roads, landing grounds, land drainage, the preservation of rural amenities and surveys to improve golf courses.
The cost of air surveys varies according to the nature and extent of the work, and is affected by various important factors. These are the position of the country to be surveyed and the distance of the operating unit from that area, the nature of the site, the services to be rendered, the weather, the magnitude and duration of the operations, the scale on which photographs or maps are produced, special conditions impossible to anticipate, and the presence or absence of ground control. The cost has ranged from under £1 a square mile for small-scale work in flat country with sparse detail to several hundred pounds a square mile for large-scale town planning.
At first air survey was handicapped, particularly in Great Britain, by the effects of the atmosphere. Since then improvements in cameras, films and filters have reduced the occasions when air photography is impossible. Moreover, when a large area such as that in Western Australia is being surveyed, the aircraft are diverted to regions where favourable weather permits the work to proceed. Infra-red photography has opened a new prospect, enabling difficulties of visibility to be overcome.
The technicalities of air survey have been mastered by a small body of men whose scientific and practical attainments have made this specialized branch of aviation of benefit to humanity. The implications of the new science are great. It can be used to avert some of the disasters which cause the loss of countless lives. The remote upper waters of rivers such as those which sometimes rise in flood in China, the thoughtless ploughing which aids soil erosion, the unsuspected sapping of coasts by the sea, can all be revealed by the air photograph to the eyes of men with the power to guard against disaster.
THE MECHANISM OF AN AIR SURVEY CAMERA automatically winds the film along after each exposure and takes photographs at set intervals of time. The length of these intervals can be varied as desired; the setting is dependent on such factors as the speed of the aircraft and the height at which it is flying. In this picture an engineer, using a stethoscope, is checking a Williamson camera for smooth operation of the mechanical parts.