166 



HYDROGRAPHICAL SURVEYING [chap. v. 



Selection 

 of Posi- 

 tions for 

 Marks. 



Officer 

 marking 

 respon- 

 sible for 

 Suffici- 

 ency of • 

 Angles. 



Tangent 

 of a Point 

 as a 

 Station. 



They should be distributed at fairly equal distances, and 

 as a general rule the more difficult it is for the coast-liner or 

 officer sounding to fix themselves, the more numerous should 

 be the marks to help them. But it is useless to put up a mark 

 in a posit'on where it cannot be fixed, and a sufficient number 

 of subsidiary stations must be fixed to ensure every mark 

 having a sufficient number of angles giving a good cut to 

 fix it. 



Projecting points and bights and bays require particular 

 attention in this respect, and every endeavour should be made 

 to secure side-shots from mark to mark. When this cannot 

 conveniently be achieved, a false station made with a sextant 

 a short distance out from the coast will frequently give the 

 required angle. 



The officer marking must think for himself whether he has 

 enough angles to fix the point ; and in case any mark cannot 

 be seen from the shooting-up station, he must get an angle 

 from some other of his marks, wliich will be then used to 

 calculate the other angles in the same manner. 



A well-defined tangent of bushes, or the liigh-water line of a 

 point, having been shot up from some fixed /^ , the line may 

 be utilized to fix another A that it may subsequently be 

 necessary to make on the point, though not on the particular 

 tangent shot up, which may be at a considerable distance 

 beyond. The required line may be found by standing between 

 the tangent of land or bushes and the A from which it is shot 

 up, and sending a man to a little distance, waving him to one 

 side or the other, moving yourself under his direction, until 

 you have the .shooting-up A, the man, and the tangent, in 

 fine from where you are standing. The line being thus deter- 

 mined, the required A can be made on it, or at a measured 

 number of feet on either side of it. This is a rough-and-ready 

 way of measuring 180°, and is frequently useful in various 

 ways when marking the coast to ensure that marks are visible 

 from any particular direction. 



If it is necessary to place the theodolite at some distance 

 beyond the particular tangent observed, the line is obtained by 

 bringing the latter in line with the shooting-up station, and 

 measuring the distance from the theodolite at right angles to 

 that line. 



