Air Photographs for Small Expeditions

Abstract

familiar with air photographs. After a brief introduction the second section describes the characteristics of air photographs?how they are taken, what they are, and how they differ from a map. Characteristics of both vertical and oblique photographs are covered. The third section describes briefly how they are used and how to find out their location, orientation, scale and tilts. The value of stereo-pairs is emphasized. The fourth section outlines how to take measurements including the making of a simple planimetric map and the measurement of relative heights but in both cases the reader is referred to text books. The fifth section describes briefly the principles of choosing and identify? ing ground control points to enable a photogrammetrist to make a more elaborate map of a small area. The sixth and most important section gives a detailed account of where and how to obtain air photographs, both of and in this country and overseas and includes a questionnaire sent to a large number of foreign and Commonwealth countries from which over 30 replies have been obtained. These are available at the Society. This article is not intended for anyone who has been trained in the use of air photographs and in particular it is not intended for the professional land or soil surveyor or cartographer. It is intended for the scientist or other worker?for example a historian or geographer?unfamiliar with air photographs but studying any features on the ground whose shape, size and distribution are important to him. Examples of such features are the natural ones such as rock structures, shore-lines, drainage and vegetation patterns, lakes, sand dunes or glaciers, and the man-made structures such as paths, towns, villages, cultivation patterns and so on. Anyone studying these is likely to want to know, and to be able to demonstrate, not only the distribution of these features, including their relation to others whether natural or artificial, but also in more detail and with more accuracy their individual shapes and structures. For all this work aerial photographs are likely to be far the simplest, most powerful and often the most revealing tool. A good example of this is the regular vegetation strips found in semi-arid areas in the Somali Republic and the Sudan (Macfadyen, 1950; Greenwood, 1957; Worrall, 1959) whose existence was not realized at all until they were seen on small scale aerial photographs. Why they occur is still not known but they may well be a significant stage in the development or degradation of marginal desert areas. The article attempts to explain in non-mathematical language, and without formulae or technical diagrams, the characteristics and limitations of air photographs, how they can be used, how to make measurements from them, and how to get hold of them. For those who wish to follow the subject in more detail there are many text books of which a few are listed in the references at the end. Characteristics How they are taken.?For most practical purposes air photographs are taken professionally from a special aircraft and with a specially designed aerial camera, usually with the lens pointing vertically downwards. It is almost universal now to take these with a nine inch square format and with either a twelve inch (30 cm), six inch (15 cm) or 3-5 inch (9 cm) focal length lens. Photographs taken with these lenses are referred to as normal, wide and superwide angle; and of these the second? the wide angle?is by far the most common. The focal length is usually given in ^ Mr. J. W. Wright is Deputy Director (Surveys) at the Directorate of Overseas Surveys of the Overseas Development Administration, Foreign and Commonwealth Office. This paper is the fourth of the series of papers on matters relating to expeditions.