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ON A STEREOSCOPIC METHOD OF PHOTOGEAPHIC 



SURVEYING. 



By H. G. Fourcade. 



(Read October 2, 1901.) 



In the method proposed in this paper, photographs are taken, 

 with a surveying camera, at a pair of points, the plates being ex- 

 posed in the vertical plane passing through both stations. A reseaic, 

 or a graduated l)ack frame, gives the means of measuring the co- 

 ordinates of any point on the plates with reference to the optical 

 axis of the camera. After development and fixing, the negatives, or 

 positives from them, are viewed in a stereoscopic measuring 

 machine, which, by combining the pictures, renders possible the 

 instant identification of any point common to the pair of plates. 

 Movable micrometer wires traverse each field, and pointings may be 

 made simultaneously with both eyes. The readings of the micro- 

 meters, referred to the reseau, give the three co-ordinates of the 

 point by direct multiplication by, or division from, constants for the 

 plates which depend only on the focal length of the camera lens and 

 the length of the base. When a sufficient number of points have 

 been plotted from their co-ordinates, contour lines may be drawn. 



TJiconj of the JSIctliod. — Let A and B 

 (Fig. 1) be the ends of the base and Q 

 and Q' the positions on the photographs 

 of any point P. 



Take A as origin and A B as posi- 

 tive direction of x-axis. 



Let (X, y, Z) be tlie co-ordinates of 

 P ; (.1;,,/, z„)(xi„f, £•/,) the co-ordinates 

 of Q and Q'. 



The equation of A P is 



Fig 



X 



X' 



Z' 



