82 HYDROGRAPHICAL SURVEYING [chap. iv. 



from their astronomical positions, the value of the imit is 

 obtained, and thence the value in miles of each side in the 

 triangulation. 



Comparing true bearings taken at one end of the survey 

 with those taken at the other end, by applying the necessary 

 angles and convergences throughout the triangulation, and 

 taking the mean if they differ, the most probable value of the 

 true bearing of any particular side is obtained, and thence the 

 true bearing of one observation spot from the other. 



Comparing this true bearing \vith that obtained from the 

 observed astronomical position of the observation spots, a 

 check is aifordcd on the accuracy of the triangulation and 

 astronomical positions, but there is no check as to scale ; this, 

 however, is of less importance than the check in bearing. 

 Great The angles of the first few triangles in a triangulation, com- 



^'^F^st'^ menced on a measured base, will require to be extra carefully 

 Triangles, observed, and the theodolite must be carefully placed exactly 

 on the spot of the mark which w411 distinguish the station. 

 For, as we shall be increasing our distances in each triangle, 

 until sides long enough to carry on the triangulation without 

 further enlargement are arrived at, any little error in an angle 

 will give a larger error in the resulting side. These first tri- 

 angles will nearly always require to be calculated, as already 

 remarked under the head of " Bases," in order to get a side 

 long enough to plot from, whatever it may be the intention 

 to do afterwards. 

 Errors in When the sides are short, any error in placing the theodolite 

 a^ks. " accurately over the centre of the ^, or directly under the flag 

 marking the A, , produces a larger error in the angles to stations 

 close to than in the angles to those at a greater distance. It 

 is necessary to pay great attention to the accurate placing of 

 the theodolite to ensure the first small triangles closing with 

 sufficient accuracy. 

 Plum- To find the spot vertically underneath a given point, such as 



mets. g^ |]^g Qj^ ^ bamboo, which it is undesirable to move and from 

 which a plummet cannot conveniently be dropped, set up 

 two boat-hook staves slightly out of the perpendicular, with a 

 weight hanging from the end of each, in such positions as to 

 subtend about 90° between them at the spot it is required to 

 find, and at some little distance from it ; the cord from which 



