Development of Air Photogrammetry: Literature Review and State of Art (Ground Photographic Surveying, Measuring and Plotting Apparatus for Ground Photographic Surveys) (Especially for GATE-Geospatial 2022)

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Surveying from photographs is by no means a new method. Soon after the introduction of photography just over a hundred years ago, it was realized that photographic images were formed according to the laws of perspective. If a photograph is taken from each of two stations at a known distance apart, any point which appears on both photographs can be located in space. Surveying cameras and plotting methods were developed by various people during the nineteenth century, but the preparation of plans was so laborious and photographic technique so crude that little impetus was given to it.

Drone Topographic Survey of a Proposed Wind Farm|Pix4D

At this stage surveying from air, photographs were no more than an idea, although the possibility of surveys from balloon photographs had been considered and experiments made.

Photographic surveying, as it is known today, has developed from the crude methods evolved during the Great War. As in so many other instances, the exigencies of war forced improvements for military needs and these improvements have since proved adaptable to the needs of peace. At first, photographs were taken by hand-camera and it was found that a series of exposures taken at, say, daily intervals of an area where the enemy was active, would show any changes clearly, and “bushes” which grew overnight were at once suspect. Camouflage had to be good to escape the searching eye of the camera; dummy trenches, effective on the ground, threw the wrong shadows in an aerial view: tracks made by a body of troops were easily discerned, and the later stages of the war were notable for the ingenuity with which attempts were made to deceive the air camera. Actually, the air photograph has made camouflage very difficult as, for instance, in the overhead cover of an artillery battery which is easily detected by stereoscopic examination of a pair of photographs.

It was realized that maps could be made from vertical air photographs, and if a sufficient number of photographs were taken from about the same height with the aircraft approximately horizontal, they could be joined together to form a “mosaic” or photographic map. Mosaics are in great demand for economic purposes, but when presented in the form of photographic maps, suffer from the disadvantages that they can never be quite true to scale and that too much detail is shown. These mosaics were much used for trench maps.

It was found that when certain conditions of flying were observed and vertical photographs taken with about sixty per cent longitudinal overlap, line maps could be produced very rapidly, the accuracy being largely dependent upon the quality and amount of ground control provided by the ground surveyor.

During the War, much progress was also made by Germany, but on different lines and with a tendency to rely upon elaborate stereoscopic viewing apparatus and automatic plotting machines, rather than upon the simpler semi-graphical methods used by the Allies.

Subsequent advances in many countries resulted in divergences in methods and technique according to requirements and inclination. On the Continent ground photogrammetry by stereoscopic methods had been developed at the same time as air photogrammetry. In Britain, the practice has generally been to simplify plotting processes as much as possible, although it is agreed that contouring with minimum ground control requires a somewhat more elaborate apparatus.

Ground Photographic Surveying

It was in this field only that practical results were obtainable until the “heavier-than-air” craft became established. In 1851 Laussedat of the French Army commenced experimental work and in 1861 produced a survey of a village near Versailles. Following Brunner՚s photo-theodolite of 1859, others improved the camera, theodolite and measuring devices. Laussedat՚s work was primarily intended for military purposes, and much of the subsequent research work in other countries was undertaken by the military authorities. In time, this led to instruments of precision such as the Zeiss Field Photo-theodolite in Germany in 1906 and the Bridges-Lee Photo-theodolite in England at about the same time, or a little earlier.

The year 1888 saw the introduction of photographic surveying in Canada by Captain Deville, Surveyor-General to the Dominion Lands. Surveys were made from ground photographs in the inaccessible districts of ale Rocky Mountains. Similar operations were commenced on the Alaskan Border shortly afterwards and were later continued by the United States Geological Survey.

European developments on a practical scale began in Switzerland in 1890, when ground photogrammetry was used to prepare plans for the Jungfrau Railway. Much of this route was through an inaccessible, mountainous country which would have been extremely difficult to survey by any other method. The real object of these early surveys was to plot in-accessible areas, but the method was not considered suitable for general use, though it could be profitably employed in mountainous country and where it was necessary to complete the groundwork quickly for climatic or other reasons. In all cases, the surveying camera was mounted on a tripod.

From then onwards until the Great War a number of photographic surveys, some by stereoscopic methods, were made chiefly by Canadian, German, Swiss and Russian surveyors, and frequently for railway survey in the difficult country. At the present time, ground photographic surveying is not much used except for large-scale surveys in the very inaccessible country, such as the mountainous regions in Canada and the Swiss Alps. Vagaries of climate have been against any extensive use of ground photographic methods in Britain.

Measuring and Plotting Apparatus for Ground Photographic Surveys

Much of the plotting apparatus employed for air surveying has been developed from apparatus originally designed for use with ground photographs. This has been particularly so on the Continent and in some cases, lack of true appreciation of the essential problems of air survey made these machines somewhat of a makeshift.

In 1902, Fourcade, a South African Forestry officer, described and later in 1902, Pulfrich of the German firm, Zeiss, described and produced a “stereo comparator.” This is an apparatus in which photographs exposed horizontally and in parallel direction can be examined and measured stereoscopically, arrangements being made for the extension of the base between the pair and for magnification. The stereoscopic measurement was on the basis of “difference of parallax” which is a measure of relative displacements of points appearing in two photographs.

About 1907, Major F. V. Thompson, R. E. , of the British War Office, added a mechanism known as a “parallax drum” which enabled distances to be read without computation and he also added a simple lever for plotting. The machine was not entirely automatic as certain readings and settings had to be made. This stereo-plotter was used experimentally during the survey of Fiji in 1908 - 10.

The first machine on these lines which gave automatic plotting was the Von-Orel Stereoautograph in 1908. Hotine considers it very satisfactory for groundwork but less so when adapted to air photographs. Sander points out that “the stereo comparator with a copying system of such a form that in bringing the image of the (floating) mark into touch with the image of the ground, the necessary motions of the slides of the stereo comparator call for motions of the slides of the copying system” which keep the plan and height in correct position.

In 1913, Kammerer put forward one of the first proposals to plot from plates inclined to one another at any angle. This solution did not employ stereoscopy, as is usually the case.

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