LEVELING 91 



BD= ffl+ycotc 

 cot a cot c 



If C' is higher than A, y will be taken as minus, and the 

 quantity y cot c will be negative. 



If convenient, select the point C' in the same horizontal 

 plane as A, Fig. 3; then, y = 0, y cot c is also zero, and 



BD = 



cot a cot c 



EXAMPLE. If in 

 Fig. 2, the angle a 

 = 17 37', the angle c 

 = 31 24', the horizon- 

 tal distance m between 

 the two positions of the 

 instrument is 300 ft., 

 and its position at C' p IG o 



is 2.5 ft. higher than 



its position at A , what is the elevation of the point B above 

 the horizontal line AD? 



SOLUTION. Substituting known values in the proper for- 

 mula and giving y the minus sign, since the point C' is above 

 the point A, 



300-2.5Xcot 31 24' 300-4.09565 



R 7) . r- - e= 1 Q ^ ftj. f f 



cot 17 37'- cot 31 24' 3.14922 -1.63S26 



BAROMETRIC LEVELING 



The variation in air pressure at different altitudes, as 

 observed by a barometer is made the basis for measuring 

 differences in elevations. As mercury barometers are not 

 readily portable, aneroid barometers are substituted. These 

 barometers are adjusted to agree with the mercurial barometer 

 at a temperature of 32 F. at the sea level in latitude 45. 

 Observations at the two stations whose diff erence in elevation 

 is required should be made as nearly simultaneous as possible, 

 because temperature and atmospheric conditions are constantly 

 changing. 



