90 



HYDROGRAPHICAL SURVEYING [chap. iv. 



Thus, in the case as in the figure above, the angles will 

 stand in the book : 



Table 



generally 



Useful. 



Extension 

 of Table. 



Rule of 

 Thumb. 



In using the traverse table, take for the course — 

 Up to 90° the observed theodolite angle itself 

 Between 90° and 180° . . 180°— the observed angle 

 180° „ 270° . . observed angle— 180° 

 270° „ 360° . . 360°— observed angle 

 — and the departure is looked out in each instance. 



The table of angles subtended by different lengths is useful 

 for other purposes. As when an angle is taken to an object, 

 and it is afterwards decided to plot a station made near that 

 object instead of the object itself, the angles to the station can 

 be corrected by it, in precisely the same manner as described 

 above, the distances and direction of the station from the 

 object being known. 



Distances or lengths, greater than those included in the 

 table, can be got by multiplication or division. 



Thus, if the angle of 18 feet at 5 miles is required, it is 

 double that of 9 feet. 



Again, if the angle of 12 feet at 10 miles is w^anted, it is 

 half that at 5 miles. 



The correction for " false station " may readily be found as 

 follows, without having to bestow a moment's thought beyond 

 appl3ang the rule, which is a matter of no small gain in time 

 when a number of angles have to be corrected. 



Rule. — Put down the theodolite reading which it is required 

 to correct (increased if necessary by 360°), and from it subtract 

 the theodolite reading of the centre of the true station. Call 

 this remainder {)• 



With ^ as a " course," and the number of feet from the 



