The New Zealand Railways Magazine, Volume 15, Issue 2 (May 1, 1940.)
The Bird's-Eye View — How Aerial Photography is Speeding-Up the Surveying of the Dominion
Although New Zealand is celebrating the first hundred years of her civilisation and settlement, it is a surprising fact that, even with the labours of surveyors dating back to before the Maori Wars, the whole of the Dominion has not yet been mapped adequately. This is because the work of triangulation with level, theodolite and chain, sometimes over the most difficult country, is a long and arduous task.
In recent months, however, in a very short period indeed, some 1,200 square miles in Hawke's Bay have been mapped on a scale of four inches to the mile, while maps of this area on a reduced scale of one inch to the mile are ready for publication.
The labours of less than half a dozen men have sufficed to perform this work in less than a year, where the task would have been formidable and prolonged under the old methods of working. Now the Lands and Survey Department is turning its attention to an area to the south of Auckland, where some 2,000 square miles of territory are expected to be mapped in equally short time.
It was the introduction of the technique of aerial surveying which has made possible this great development in the mapping of the Dominion. The work is being carried on in this country with the assistance of the Royal New Zealand Air Force, which has supplied the machines and pilots for the aerial work.
That such duties provide valuable training for the aviators is confirmed by the fact that the Allies aerially surveyed and photographed the whole of the Siegfried Line in the first week of hostilities on the Western Front. The man in the street, however, can gain a better idea of the value and rapidity of this method of surveying when it is revealed that in Canada, in 1929, an area of country almost the size of the British Isles was completely mapped in one year.
The impetus to aerial photography arose out of the Great War. The Royal Air Force began to take photographs from the air, in 1912, shortly after the aeroplane became practicable, but the stress of conflict was responsible for bringing the technique from the experimental stage to a maturity which won for the Royal Air Force the title of the “eyes of the army,” not because of the acuity of vision of its observers, but through the wealth of accurate information supplied by the aerial cameras.
Not every type of aeroplane is suited to aerial photography. Plenty of power is needed, so that the craft may climb quickly to high altitudes, also large cruising range and good visibility from the pilot's seat. Twinengined aeroplanes, with the cockpit well forward and between the engine mounts, are best for the work, which in New Zealand is being conducted with Airspeed Oxford and Monospar machines.
The aerial camera itself is a specialised instrument, and many steps were necessary to bring it to its present state of efficiency. Simplicity, compactness and lightness in weight are essential. The first models were held in the hand, but nowadays the camera is built into the aeroplane's fuselage, pointing vertically downward and driven either by wind force acting upon an external propeller or by an electric motor.
Although multi-lens cameras, sometimes taking as many as nine separate photographs at the same time, have been used in some countries, New Zealand has followed the more usual practice of using a single-lens camera with an f4.5 lens of 8 1/4 inches focus which is standard all over the world. On some occasions, when the use of the large film, 9 inches by 7 inches, would be wasteful, a smaller camera covering plates five inches square can be employed instead.
The actual technique of taking the aerial photographs is not the least interesting portion of the survey. The flying is done only on clear days, when no low-lying clouds are likely to mar the negatives. The pilot rapidly climbs to the normal flying height of 16,000 feet, at which altitude the photographs secured will be on the scale of 30 chains to the inch.
That height attained, he takes up his course and sets “George,” the automatic pilot, in action. Then he looks into the ground-glass focusing screen of the drift-sight and notes the time taken by some prominent object on the ground to pass between two lines drawn across the ground-glass. That interval is the time which page 18 must elapse between successive exposures if a continuous record is to be obtained. Setting the camera to make its exposures automatically at that interval, he begins his survey flight, which in New Zealand is generally made in the East-West direction because of the prevailing westerlies and the northerly turning errors of the compass.
The pilot's chief concern is to maintain his course and keep his 'plane level. Manual piloting can be done with a normal tilt of about two degrees, but such a tilt would create many troublesome computations and corrections by the mapping staff and greatly lengthen its labours.
Fortunately “George” is an even better pilot than any human could hope to be, for theoretically he can maintain the set course without the tilt becoming greater than a quarter or half degree.
Having set his camera and seen that “George” is working properly the pilot glances at his dashboard. As the camera is about to begin exposing a strip of film a warning red light flashes on the dashboard and the work is on.
Every thirty seconds the light flashes again and a few seconds later the camera exposes a new portion of film to the landscape below, while all the time “George,” watched by the pilot, keeps the aeroplane at 16,000 feet altitude, on a straight East-West line and level.
There is 120 feet of film in the magazine, specially prepared for aerial photography, and when over 200 pictures have been secured the magazine has to be changed. This operation requires but a few seconds, and the aeroplane is turned back on its course to cover the strip of land contiguous to that just photographed. On a good day the 'plane can cover an area of 500 square miles.
Flying at 16,000 feet and at 140 miles an hour, the lack of oxygen soon becomes noticeable. After about an hour's flying the pilot is tired and tends to lose his ability to concentrate. Yet it is not often that he is called upon to repeat a flight because of errors of flying or faults in the negatives.
When the strip of film is developed and printed, one might at first think that a serious error had been made, for each successive picture contains no less than 60 per cent. of the detail recorded on the preceding negative, while some 25 per cent. down the side of a strip is to be found also on the side of the next strip. This overlapping of images, however, is one of the most valuable features of aerial photography, enabling a stereoscopic relief to be obtained by the mapping staff. Along the side of the film, like the sound track in a talkie film, is a strip upon which images of the clock and altimeter are registered at the moment of each exposure.
In some countries, notably Canada and India, oblique photographs are taken on aerial surveys, but this method is only practicable in more or less flat countries or for small-scale exploratory maps in inaccessible country. The oblique photographs can be transformed in a transforming camera to the plane of the vertical photograph, while simplified plotting schemes have been devised enabling small-scale maps covering large areas to be turned out in very short time.
The English method, known as the Arundel process, is used in New Zealand. It demands that the photographs must be true bird's-eye views looking vertically downward, and is capable of much greater accuracy than is possible in the oblique method of charting.
The topographers in the survey office place two successive photographs in a stereoscopic machine designed for the purpose and examine the “fused” images through an eyepiece. The heights of the land at a few points on the prints have already been determined by ground surveys. The stereoscopic image shows a floating, horizontal grid which can be raised or lowered to rest on one of these points whose height has already been determined by the ground control staff. When set to a desired level, the topographer sketches in the points where the grid cuts the stereoscopic model, and so marks on the photograph the contour at that level. A slight turn of a screw shifts the grid to the next contour level, whose outline is again drawn in. Thus the whole of the overlap is quickly covered with accurate contour lines.
The stereoscope cannot be used in determining the positions of objects. The photograph, though map-like, is not a true map, as would quickly be discovered if a series were joined together. Hills, being nearer to the camera than valleys, are distorted outward from their true positions, while low-lying land is found nearer the centre than it should be.
Points common to the two photographs have to be selected: a few if the terrain is relatively flat, many more if it is hilly. Radial lines are drawn from the centre of the plate through the selected points which are common to both photographs, and the point where the two lines intersect indicates the true position of that object with an accuracy sufficient for all practical purposes, for no feature on the maps is displaced by a scaleable distance.
Many types of corrections become necessary, because of tilt in the aeroplane or slight wanderings off course. Consequently the examination of the films secured in a fortnight's flying by one pilot can keep a survey staff of ten busy for a year. At the end of that time, however, they have produced a map which might have required more than ten years' work under surface-surveying methods, and they have the consolation of knowing that if greater detail should ever be required, or maps printed on a larger scale, they have only to turn to these permanent records to produce what is required. The flying will not have to be done again.
Mosaic maps, though sometimes produced, are not popular with the mapping staff. Such maps are built up from small portions of adjacent photographs to provide a complete aerial view of a city or some other region requiring special study. In New Zealand a mosaic has been prepared page 19 of the Hutt development area, and occasionally, similar maps are made for farmers requiring charts of large properties.
Owing to the distortion inherent in a wide-angled lens, these pictures cannot be fitted together edge to edge with satisfactory results, as can be done in a jig-saw puzzle. If that course were followed the fitter would receive some unpleasant surprises when he got to the edge of his jig-saw. The shortest boundary of the farm, for example, might appear in the finished result longer than any other boundary, while portions of the land shown near the edge of the mosaic might have no actual existence. Mosaics are much better fun than jig-saws, if difficulty in piecing them together is any indication.
Apart from the rapidity with which whole countries can be mapped under aerial surveying, and the toil and labour obviated in mountainous country or barren regions, the bird's-eye view has been responsible for many notable discoveries. In Australia, in 1930, a large salt lake, 70 miles long and 30 miles broad, was discovered by aerial survey in a place where its existence had not previously been suspected. Likewise, the Hawke's Bay survey has revealed earthquake faults running for miles over the country, with one side as much as twenty feet higher than the other, which from surface level would pass unnoticed. Their mapping from the air may provide valuable data for future research on earthquakes in the Dominion.
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Similarly the historian would be interested in the aerial views of ancient Maori pas, the earthworks of which stand out prominently in aerial photographs.
The survey of Hawke's Bay resulted in three sheets of the new topographical survey being produced, while another six will come from the flying now being done over the South Auckland area. To cover the whole Dominion on the present scale of one mile to the inch no fewer than 360 sheets will be required, but it is safe to predict that, in a very few years, so rapid is this new method, there will be no blank spaces on the map of New Zealand.
Aerial photographs are also being used in geology, mining and prospecting, being particularly valuable in country which is difficult of access and wooded areas where ground vision is very limited. Even town-planning has been facilitated in New Zealand and overseas by a study of mosaics built up from aerial photographs.page 20