Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . ement of angles. There are a verticaland a horizontal cross-hair in the focal plane of the camera,and it is fitted with a magnetic needle inside of the box,and a scale, so placed that, when the exposure is made,the magnetic declination, the scale, as well as the intersec-tion of the cross-hairs, are all photographed on the platecontaining the view (Fig. 91). If the instrument has beencarefully leveled, the horizontal cross-hair becomes the hori-zon line, a
Topographic surveying; including geographic, exploratory, and military mapping, with hints on camping, emergency surgery, and photography . ement of angles. There are a verticaland a horizontal cross-hair in the focal plane of the camera,and it is fitted with a magnetic needle inside of the box,and a scale, so placed that, when the exposure is made,the magnetic declination, the scale, as well as the intersec-tion of the cross-hairs, are all photographed on the platecontaining the view (Fig. 91). If the instrument has beencarefully leveled, the horizontal cross-hair becomes the hori-zon line, and the vertical cross-hair the center zero line, towhich angular measures are referred in the office computa-tions. A group of views are taken at each station, abutting PRINCIPLES OF PHOrOTOPOGRAPHY. 297 one against the other, and the angular distance between eachis noted by the reading of the horizontal plate of the camera,horizontal angles being also read by a small theodolite or bythe camera to the more prominent peaks. The objects represented in perspective are of an irregularghape and at various distances from the camera. If the. Fig. qi.—Bridges-Lee Photo-theodolite. picture or image of the object is a true per::ipective in aplane, it is possible to construct therefrom a geometric pro-jection of the object in a plane at right angles to the pictureplane. This, providing the distance and the relative positionof the point of view be known with reference to the pictureplane, and providing views have been taken from a sufficient 298 PHOTOGRAPHIC SURVEYING. number of stations to surround the irregularly formed objectsviewed. Plioto-topograpJiy is, therefore, the art of recon-structing geometrically horizontal projections from perspec-tive views. The process of this reconstruction consists inplatting the skeleton triangulation as obtained by angularmeasures with theodolite or the horizontal circle of thecamera. The photo-topographic survey should be preferablypreceded by a pri
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