PHOTOGRAPHIC SURVEYING (from Gk. ¢7e, phis, light -1-- -yectOetv, graphein, to write). In recent times the art of photography has found its way into surveying, an ttpplieation due chiefly to the French. Its advantage was at once rec ognized for military and topographical purposes, but its usefulness for accurate and extensive surveys is still quite limited, owing to- the im perfections of the photographic lens. The chief instrument is a view- camera provided with cross wires and an apparatus for leveling. A survey is effeeted by selecting at least two stations, and measuring the base-lines between them. Then at each station a series of plates are exposed, the camera 'wing turned at each successive exposure sufficiently to obtain for the series a continuous photograph of the area to be surveyed. The method of executing a map and of calculating heights and distances from photographs is best obta he'd from works on the subjeet. but a gen idea may he gained from the following. since the images of the cross-wires are photo graphed on the plate, every point of the field appears in its proper position in relation to the four quadrants of the plate. Thus in Fig. 1 1111:
is the image of the horizontal wire, VA' the image of the vertical wire, and P the image of a church spire. The lines a and b can be measured in the photograph, and the focal distance f being known. line d can be computed, and when the distance ()A has been determined from the map, the height of the spire above the level of the instrument 1; given by similar triangles. To plot a survey, the angular distance of any point from the hase-line is determined by a photograph taken at each end of the base. in Fig. 2, P represents the plate of the camera at station A; B', C' being the images of station B and the point C: the angle B' OC' can be determined from 13' C' and the focal length f. Then any point of the map may be plotted as follows: Lay oil the base-line to a desired scale. and draw lines making angles with the base equal to those computed. These will intersect in a point on the map corresponding to the point in the field. Consult Deville, Photographic Survey ing (Ottawa, 18951. See SURVEYING.