1S92.] ^'J^ [Bachc. 



to execute a survey, and the area comprised by them is pictorially 

 duplicated from one or more stations. It is always desirable that 

 the same objects shall be seen, if good intersection of rays can be 

 secured from the different pictures, from three stations instead of 

 two, because an error in one of the azimuths at the end of a single 

 base, which of course gives only two lines for an intersection of 

 rays, would vitiate a whole survey, whereas, with two bases, involv- 

 ing three points of view, and the intersection of three rays, accu- 

 racy throughout a survey receives a crucial test. The adoption of 

 this plan, which is like that employed in ordinary triangulation, is 

 also desirable on account of its securing accuracy of plotted re- 

 sults ; because graphical differences in the positions, as given by 

 the intersection of only two lines, are virtually eliminated by 

 obtaining for intersections the mean positions as derived from three 

 lines. 



The survey, so far as the instrumental part of it is concerned, 

 being complete, it only remains that the plotting of it shall be 

 done. The base line being laid down to scale on paper, lines are 

 drawai from its termini, at the angles with it represented by the 

 azimuths of the lines of sight as determined there on the ground. 

 On this representation on paper of the lines of sight, at the respec- 

 tive plotted stations, are placed, at right angles, printed on thin 

 paper, the photographs taken at the two stations, in such manner 

 that the individual plotted line of sight shall point on the photo- 

 graph upon the representation of the object upon which the real 

 line of sight was directed in nature, after that representation shall 

 have been vertically projected on the horizon line of the photo- 

 graph, and that the horizon line of the photograph shall be distant 

 from the individual plotted station by the focal length of the par- 

 ticular camera that was used in taking the pictures. The eye then, 

 placed in position over a plotted station, and looking at a photo.- 

 graph corresponding to the view taken from that station, sees, as 

 already demonstrated, that view under precisely the same angular 

 effect as the view is presented by nature on the ground. Conse- 

 quently, as angles formed by rays of light with the base line are 

 given truly in nature, are also given truly by the camera, and are 

 now given truly as plotted on paper to become a map, the intersec- 

 tion on that paper of these rays, as proceeding from the pictorial 

 representation of the objects from which they are derived in nature, 

 after their pictorial source has been vertically projected on the hori- 



