THE PRODUCTION OF “MOSAICS”, which consist of a number of related air photographs of the ground. The photographs are all to the same scale and are joined together on a support of plywood or stout linen. These “mosaics” are invaluable for obtaining a general view of all the details of a locality.
AIR photography has become an important science chiefly because there is everywhere an unceasing demand for maps. Maps are a graphical way of presenting facts. They are valuable because the graphical system gives vast amount of information in a remarkably small space. Photography has been called on to lend its aid in mapping, as in almost every other important industry. Photography was occasionally used on the ground and from balloons in the making of maps many years before aeroplanes came into being. But it is only since the advent of the modern aeroplane that photography has played any considerable part in map compilation.
The basic methods of air photography resemble those used for other kinds of photography. A camera mounted on a platform has its lens pointing through a hole to the ground vertically below the aeroplane. The film is carried on spools, just as it is in an ordinary roll-film camera, but each length of film accommodates 200 exposures, and the film occupies a horizontal position. The platform that serves as a camera-mounting is provided with levelling devices, and shock absorbers eliminate vibration.
Exposures are made and the film is wound on at regular intervals of time determined in sympathy with the speed of the aeroplane. An interval of 10 seconds is normal, and the shutter speed normally used is about second. In this way it is arranged that each photograph shall include in its field a portion (generally about 60 per cent) of the view embraced by the previous photograph. Every part of the ground over which the aeroplane flies is thus photographed twice. This is done deliberately, so that the valuable properties of stereoscopic vision may be brought into play later in the drawing office.
Various instruments - an altimeter, a watch, a “Veeder” counter and a small spirit level - are all housed in the camera, and images of their dials are automatically recorded on the margin of the film at each exposure, through a number of small supplementary lenses. An optical aiming sight is used by the photographer to check the navigation and set the time, interval. An electric motor is used to wind on the film and set the shutter between exposures. An electric control box continues to release the shutter at the desired time-interval without further attention once it has been set and switched on, until it is switched off at the end of a run.
The task of the pilot flying up and down straight lines sounds monotonous, but it needs specialized skill and concentration. He takes the aircraft to the predetermined height, gets over his starting point, and sets the aircraft on the desired compass course. He then has to hold everything constant. Flying height must not vary, flying speed must be maintained, the compass course must be held without any sensible error, and the aircraft must be kept on an even keel both fore and aft and laterally, because unwanted tilts introduce difficulty into the various processes of mapping from the photographs. This will be explained in a later chapter.
At the end of the run the pilot turns the aircraft round and flies back along a path parallel with his previous run. In this manner he covers the whole territory with a series of parallel strips of photographs. The direction and strength of the wind have to be carefully observed and measured and a correction for drift is applied. Good survey pilots need to keep their whole attention on their work; but even their best efforts are not as near perfection as the drawing office staff demand, and gyroscopically stabilized controls for the aircraft and for the camera are now being perfected for this critical work.
Vast areas of the Earth’s surface are still without detailed maps. As growth and development without an ordered plan must lead sooner or later to waste and confusion, and as adequate maps are the prerequisite of ordered planning, map-making in undeveloped countries has acquired a special significance. It is in such countries that air photography has achieved some of its most valuable results. It is still true that economic development largely follows the railway, and in any development scheme it is essential that the plans of a new line should be produced for the best possible situation after thorough exploration and study of the country through which the line is to pass. Air photography is proving of considerable use in exploration and in deciding exactly where new railways are to be built.
The study of the air photograph has brought into being an entirely new technique in geological exploration. Every civilized country is at present taking stock of its natural resources, and the search for minerals, such as gold, iron, copper and oil, is more intense now than ever before. Searches in the past have been somewhat haphazard, and lucky strikes have sometimes rewarded somewhat unskilful prospecting. The prospector on the ground finds it difficult to form a true idea of the sub-surface geological structure. His view is limited, and it is not easy for him to obtain a true perspective over large tracts of country. Since air photographs have been available, however, it is much easier for him to correlate the surface features with the underlying body and the whole terrain can be studied as one comprehensive unit.
EARTHWORKS IN SOMERSET seen from the air. These earthworks, situated on Walton Common, near Clevedon, indicate the position of what was probably a cattle enclosure of the Iron Age. Air photographs assist the archaeologist because they show details - such as slight differences of colour - which are not discernible from the ground.
Oil in the past has been discovered largely by experimental drillings in the neighbourhood of seepages; but nowadays geologists know that oil is generally associated with certain types of rock of a known age. Oil companies employ armies of geologists to study the Earth’s surface and from it to map the sub-surface geological structures. The study and the search become much simplified when a complete picture of the surface is available. There is no quicker way of obtaining a complete picture of the surface than by photographing it from the air. Many thousands of square miles are now being systematically photographed in many parts of the world, solely in the hope and belief that the results will facilitate the location of oil and other invisible subsurface minerals.
Many countries owe a large portion of their wealth to the bountiful manner in which Nature has endowed them with extensive forests of valuable timber. Unless these forests are to be wastefully exploited some more or less rigorous control is necessary. Forestry officers are appointed to see that the future is not needlessly sacrificed to present greed. Stock maps are necessary to the forestry officer in his work of conservation, and air photography is of great use in compiling such maps. The trees can be counted, their type can be determined, their height and timber content can be estimated and disease can sometimes be detected from air photographs. Timber poaching on a large scale is not unknown in some parts of the world - for example, in the teak forests of Burma - but the misdeeds of the poacher are immediately apparent from the photographs, and the task of checking the poacher’s activities is simplified. At the same time the photographs help in choosing the most economical routes for sending the cut timber down to civilized regions. Of the many purposes for which air photography is used, military science ranks high in importance. The more knowledge a military commander obtains of the territory upon which he is to fight the better he is able to arrange his plans. If he is to fight in a strange country he sends out at the first opportunity his reconnaissance aircraft to bring back air photographs. These photographs will indicate the natural obstacles to his advance. Throughout the campaign air photography is used for a variety of purposes.
Air photography developed almost from zero in the war of 1914-18. The battlefields were photographed day by day to record the effects of bombardment, to detect machine-gun nests and hostile batteries, and to prepare trench-maps of the constantly growing defence systems. The concentration of troops could be inferred from the number and movement of trains in each sector, and all the activity behind the enemy’s lines was recorded daily by air photographs.
One of the most valuable aspects of air photography is the facility with which it can be applied to the revision of out-of-date maps and plans. In Great Britain there exists an excellent and reliable map system. The 25-in-to-one-mile Ordnance Survey plan is a masterpiece of the surveyor’s craft. Yet if it is not frequently revised it loses a large part of its value. The necessary cycle of revision was inevitably held up during the war years and was not maintained at its proper pace during the years of depression that followed.
This interference with the normal cycle of revision had unfortunate consequences, and the out-of-date condition of the plans became greatly intensified with the enormous amount of building that has been witnessed in recent years.
The people charged with the revision of plans have to face a problem intensified by arrears of work and by greatly accelerated changes. These changes are not peculiar to Great Britain, but find a parallel in every civilized country. Air photography has been called in to help.
THE MANAWATU RIVER, NEW ZEALAND, nearly meets itself at the neck of the loop. Changes in the course of a river may produce floods; but often the changes can be forecast from air photographs of the river. In air survey it is possible to take such photographs at a speed of more than a hundred miles an hour.
This problem of revision is the more urgent because a halt has been called to promiscuous development. Town-planning schemes are being introduced everywhere to prevent the chaos that would result from a continuous stream of uncoordinated effort. Town-planning authorities demand up-to-date plans upon which to work. The manner in which air photography has proved itself of value in the making of war-time trench-maps demonstrates its potentialities for peace-time mapping. An air photograph taken on a horizontal negative with the lens-axis pointing vertically to the ground below is in itself nearly an exact plan, and it can quickly be brought to any desired scale by simple optical methods. As every exposure covers about a hundred acres of ground at the desired 25-in scale, it is clear that the photographs can be of immense value to the reviser of maps and plans. When, in addition, it is realized that the speed of the aeroplane permits an exposure to be made every few seconds, it is evident that a few hours of photographic flying will provide as much data for the drawing office as would require many weeks of work with theodolite and chain on the ground.
The roads of today do not suffice to carry in safety all the motor traffic that wants to use them. The only cure of the trouble seems to be the building of more and more roads, and air photography comes forward again to find the best routes for the new roads.
In some parts of the world the land owned by the peasants is almost their only wealth, and taxation is based upon the area of each man’s holding. Some of the plots are small, but may nevertheless be valuable because of their fertility and of the fact that three crops a year can be had from them. This is common in Egypt and to a lesser degree in India. The survey and measurement of the area of each small plot by old-fashioned methods are slow and laborious, but air photographs greatly facilitate the work, by permitting the areas to be computed rapidly in the office with a planimeter or similar device. Some of the most important air survey work yet undertaken owed its inception to the possibility of using it to help in collecting national revenue.
Fight Against Erosion
The proper maintenance of river banks and channels is work of first importance. Air survey is valuable, not only for making plans of the rivers, but also because of the pace at which the aircraft can gather the necessary data. River engineers often want a river plan that shows the state of the river simultaneously over large areas. Air survey is able to provide the records speedily. The damage caused by silt or erosion can be quickly discovered and means of combating it can be devised.
In a totally different sphere - archaeology - air photography has produced some noteworthy results. The elevated viewpoint of the air survey camera and the photographic ability to distinguish slight differences of colour and tone have led to some remarkable archaeological discoveries. Many records of pre-Roman activity in Great Britain have been found with the air camera, but perhaps the greatest discovery of this kind was recorded near Samarra, Iraq, when air photographs revealed traces of a prehistoric city twenty miles long and two and a half miles wide.
The term “mosaic” is often used in connexion with air survey. A “photo mosaic” is made by joining together a number of individual photographs into one complete picture-plan, just as a mosaic pavement is made by joining together a number of cut pieces of stone or marble to form a complete pattern.
The mosaic is the most elementary form of air-survey map, but for many purposes its ability to show continuity of feature renders it extremely valuable.
Further chapters in this work will deal with specific aspects of the subject of air photography.
DISCOVERED BY AIR. This wall, which can be seen crossing and recrossing the dry bed of a tributary of the River Santa, runs back into the Andes, Peru, for a distance of more than thirty miles. It was, apparently, built by an ancient coastal people.