Page 221 hydrography 3338 



sounding line will plot as a curve, and a large jum'p will occur when, at a considerable 

 distance from the buoys, one or more of the buoys comprising the fix are changed. 



For a method of strengthening the azimuths between adjacent buoys in a line of 

 buoys located by sextant cuts, see 2552. 



The difficulty of observing sextant angles between survey buoys beyond sight of 

 land is considered in 4522. 



3338. Inclined Angles 



Where a sextant angle is observed between two objects which do not lie approxi- 

 mately in the same horizontal plane as the observer, the observed angle must be cor- 

 rected before it is used for plotting. Where one of the objects is at or near sea level 

 and the other at a sufficient elevation to cause an appreciable error, the inclined angle 

 may be corrected by the use of the graph in figure 56. This graph is based on the 

 formula 



cos Vo 



cos Vc.=- 



cos h 



in which Fo= observed or inclined angle, A = angular elevation of the elevated object, 

 and yc=horizontal or computed angle. (See also 941.) 



To find the correction to an observed angle, enter the graph at the left-hand margin 

 with the altitude angle as an ordinate and from this point extend- a line horizontally 

 until it intersects the curve representing the observed inclined angle, interpolating if 

 necessary between the curves shown on the graph. The abscissa of the point of inter- 

 section read on the horizontal scale at the bottom of the graph will give the correction 

 to be applied. The correction is subtractive for angles less than 90° and additive for 

 those greater than 90°. Note that the above formula and this graph are only applicable 

 where one of the two objects is elevated. 



If both objects are elevated sufficiently to require correction, the correct horizontal 

 angle may be obtained from the following formula: 



cos Vc- 



cos Vo— sin hi sin ^2 



cos hi cos /12 



in which hi and ^2 are the angular elevations of the two objects. Logarithmic computa- 

 tion will be facilitated by converting this formula into the following equivalent formula: 



m 



. oy converting tnis lormuia mio tne lonowu 



cos ]{ Fc= Vsec hi sec h2 cos S cos {S— Vo) 



which s=YA^i±h 



A convenient form for using the formula is as follows: 



Vo 



hi • sec 



hi sec 



2) : 



S - cos . 



S-Vo - COS- 



2)1 



KFc _ _ - 1 COS. 



K 



