598 



PROCEEDINGS OF SECTION H. 



Thus the working focal length of the lens is 8-475in., or nearly 

 8Jin. Several horizontal angles were determined from the photo, 

 with this factor, and they agreed witliin a few minutes, more or 

 less, with those taken with the theodolite. The angle of elevation 

 to a prominent hill was also found, as follows : — Height above 

 horizontal line = 'BSin. ; divided by focal length, 8-475in., gives 

 tan. of 4° 15', only a minute different from the observed angle. 



FIC. I 



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HottizoivrAL Ancles; FBsmffJifom. 



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 FROM PHOTO. 



TrueAnclePoint. 



The simplest application of photogrammetry to vertical angles 

 is to determine the height of a telegraph post from a photograph. 



