On a Stereoscopic Method of Photographic Surveying. 35 



The contour lines should then, in the case already considered, be- 

 accurate to 0*25 mm. on slopes greater than 15°, but the actual 

 accuracy will be reduced to some extent by the uncertainty of the 

 correction for refraction. This correction, combined with that for 

 curvature, can be applied at sight from a small table with 7/-argu- 

 ment. 



By reducing the base, pairs of photographs may be taken within 

 a confined space, as when mapping hidden valleys. The method 

 can also be combined to any extent with the ordinary methods of 

 photographic surveying. It would be of particular advantage in the 

 mapping of large areas of mountainous country. 



