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Aviation History
1924
1924 - 0134.PDF
SOME FEATURES IN THE PRESENT POSITION OF AERIAL PHOTOGRAPHIC SURVEY By H. HAMSHAW THOMAS (Concluded from page 122.) " AERIAL survey by vertical photographs approaches the ideal survey at least in one point—it does or can provide the greatest amount of detail about the surface of the ground. The amount of detail is far more than it is ever humanly possible to obtain by a ground survey where any large area has to be covered. It may be possible to see on the photo graphs, taken from a height of 10,000 ft. objects on the ground as small as 2 ft. across, if they project slightly and cast a shadow. This knowledge of surface detail may in places be of little importance, but in other places may have very great value. Its value may be estimated from two points of view—firstly as increasing our scientific knowledge of the earth, and, secondly, as having an economic importance in connection with the development of communications, of irrigation and water supply, of agriculture, of forestry and of mining operations. As man pushes out further and further into uninhabited country, he needs surveys of the country, and if he can get a primary survey of topographic position, together with all the detailed information necessary for development, much labour will be saved. The British Empire at the present time contains many regions which are waiting for an aerial survey. To quote from the Report of Canadian Survey :— " ' The settled part of Canada, of which more or less imperfect maps are available, is confined to a fringe or belt along the southern boundary. The northern part, of immense extent, is very little known. Roads do not exist, the only means of communication being by dog-train in winter and by the lakes and rivers in summer. Traverses along the waterways furnish the basis for such maps as we have. In the best of these traverses, the angles are measured with a theodolite and the distances by stadia or with a range finder. The surveyor completes his plan by sketching such details as he can see from the level of the water ; what is beyond the shores or the islands is necessarily omitted.' " Such a survey is no doubt useful as a preliminary recon naissance, but if any development is to be done the country will probably have to be again surveyed for the purpose. Such work may be long and difficult, and its difficulty may hinder the progress of development for many years. Every one who has had any experience of finding a way through forest regions without tracks knows how difficult it may be, and yet at the same time there may be natural roads through at least part of the forest zone if they could only be located. An aerial survey in a densely-wooded area presents no more difficulty than a survey over an open area. It may be carried on at the rate mentioned above, provided that landing grounds can be found in the vicinity, and in many places landing can be effected for seaplanes on lakes or rivers. Such a survey shows at once where are the natural roads through the forest or at least which are the shortest routes through. It is thus a most important preliminary to the construction of roads or railways. " Similarly, in hilly districts aerial survey, by reason of the complete and rich record of topographic detail, will often show the engineer the best routes along which communica tions may be built. I came across an example of this in North Central Palestine. The plains of Central Syria are divided from Palestine proper by a stretch of very hilly country stretching between Hermon and the sea. At no place are the hills very high, but there no road has ever been made through this country to link up the more prosperous communities to the north and south ; communications involve a very considerable detour to the east or to the west. And yet through this region there exists a natural route difficult to locate on the surface of the ground, but obviously the best means of traversing the hills. This is a dry river valley which once led the drainage of Central Syria into the Jordan Valley. It is not shown on the map prepared by Conder and Kitchener, but was evidently discovered during the War when the making of a road along it was commenced. It was well seen on aerial photographs. " Aerial survey shows every detail in the drainage system of an area, and may thus be of great use in connection with water supply, the sighting of reservoirs and irrigation. In Canada in 1922 much work was done for this purpose." " Aerial survey has contributions to make to the science of botany with its more practical sides of agriculture and forestry ; to geology with its obvious application to mining and other problems ; to archaeology with its general appeal to the human mind ; and, of course, to the all-embracing science of geography. Let me endeavour to point out a few of the points in this connection which have come to my notice. " Botany.—A most important feature of the earth's surface is provided by the vegetation which covers it. The study of this vegetation and its distribution, the causes which underlie the occurrence of certain types of plant commodities in one spot and the growth of similar or dissimilar vegetation in another locality, form the subject of much investigation by botanists today. It may be that some of the plants in the native vegetation have an ultimate economic value, and it may be that the ground occupied by certain types of native vegetation may be readly cultivated or utilised by man, or it may be that the vegetation is a factor which is influencing the shape and extent of topographical features such as coast lines. But in other cases the study of plant distribution may have no immediate practical application, and serves only to add to our knowledge of the workings of nature. " It can be readily understood that aerial survey forms a ready means of mapping the extent and distribution of forest, but it may come as a surprise to some to know that in many cases it also enables us to accurately ascertain the numbers of certain trees in any area, the size of those trees, or at least the relative size ; further, that after a little practice we may recognise certain kinds of trees when we see them on the photographs. Again, aerial survey records the exact extent of areas in the forests which have been burned or cut down, and of others in which young timber is again growing up. " A forest tree is a big thing, and it is not surprising that in photos. of coniferous forests we can often distinguish each individual full-grown tree, but even when the vegetation of the ground is composed of tiny plants, the all-seeing eye of the camera may be able to record their presence. " I have already referred to the survey of Blakeney Harbour in Norfolk, and I may here mention some points of interest which were derived from the study of the photographs taken at this spot. " The ground surveyed was to a considerable extent covered with the natural vegetation of maritime areas. It included sand dunes with their tufted grasses ; much of the area was salt marsh with a variety of plants, varying from tangled masses of the sea-weed Pelvetia ; tracts covered with the sea lavender ; sward more or less carpeted with grasses, to a good deal of bare mud without any plants. The great shingle spit bears the important bushy Suaeda fruticosa, which helps to stabilise it, with other occasional plants. We found that our aerial survey provided quite a good map of all this vegetation at the same time as it gave the complicated topographical detail. After a short time on the ground with the photographs one could soon recognise how each type of vegetation was depicted, and the relation of several of the vegetation types to the topography was at once shown. " For example, we can see how the incipient dunes are being colonised by the marram grass, any tuft over 2 ft. across being visible on the prints ; the grass on the younger dunes is still in isolated tufts, but on the older dunes marram and other grasses have grown together, forming a more or less continuous covering and anchoring the blowing sand. The shingle-binding Suaeda is clearly seen as a line of darker dots, and tracing it along the bank we come to an area in which it has died out. Such an area will be most liable to the attack of the waves in a storm, and, in fact, after the first survey was made a very high tide and rough weather broke through the beach at this point, destroyed the concrete sea wall and flooded the pastures behind it. We see the early stages in the colonisation of bare mud by vegetation, a process which will probably end in the replacement of mud flats by a luxurious sward." " The topographical side of this work merits more atten tion than I am here able to devote to it. Blakeney Point is one of those areas where the coast line is undergoing rapid changes, so rapid, in fact, that the charts made by ordinary methods soon become out of date. The changes which took place between 1913 and 1921, especially the build ing of a new spit nearly a mile in length, are shown in two 134
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