Planetabling from the Air: An Approximate Method of Plotting from Oblique Aerial Photographs

Abstract

T HE method of oblique aerial surveying described in this paper' offers a means of making small-scale contour maps at a relatively low cost as compared with either ground survey or conventional air survey methods. Many sections of the earth's surface that are not in a state of settlement or economic development requiring largescale maps are important from the standpoint of cartography and physiography. For instance most of the coastal regions of Mexico and Central and South America even along routes regularly traversed by the air transport lines are still unsurveyed except for the delineation of the coast line and the location of a few spot heights. A broad strip of these coastal areas could be mapped by the method proposed in sufficient detail for all present purposes, the necessary photographs being taken during the regular flights. Aircraft, while rapidly becoming an essential part of exploration equipment, still are used for the most part for transportation and reconnaissance work by observation only. The Oxford University Arctic Expedition of I924 made use of oblique aerial photographs in constructing the maps of the expedition.2 It is also understood that the Survey of India has done extensive reconnaissance air surveying in mountainous regions. Nevertheless it has rarely happened that aerial photographs have been used for the construction of contoured maps on exploratory work. The reason for this seems to be that the money generally available for a reconnaissance survey does not allow for the cost of the immense amount of labor required for a thorough utilization of the wealth of detail provided by vertical or oblique photographs taken in the conventional way. Again, in order that the height data contained in the photographs may be utilized it has been generally considered necessary to provide extensive ground control, in which event it may be cheaper and more efficient for the purposes in view to make the hypsometric part of the survey altogether by ground survey methods.