126 



HYDROGRAPHICAL SURVEYING [chap. v. 



Double 

 Calcu- 

 lated 

 Angles. 



Station 

 Pointers 

 for Chart 

 Room Use 



Small 

 Angles de- 

 sirable for 

 Plotting. 



Plotting 

 by Dis- 

 tances. 



It is advisable to pursue this system even in the case of 

 peaks that are not visited, if they are of importance in the 

 survey, and are hkely to be much used for purposes of 

 " shooting back." 



Calculated angles being entered in this manner, they may be 

 used in their turn to obtain a second calculated angle, with the 

 further advantage of giving a choice of zeros from which to lay 

 down the calculated angle, and thus to select a zero making a 

 smaller angle for plotting than if obliged to use the original :'A 

 as a zero from which the angle was calculated. 



A thorough grasp of the foregoing greatly facilitates plotting 

 in certain cases, and provides resources to fall back upon when, 

 from any cause, observed angles become scanty. 



A large pair of station pointers carefully tested for errors is 

 very useful for lajang down small angles (under 10°) for topo- 

 graphy, etc. A single peak on a distant range of hiUs being 

 fixed by chords, it can be used as a zero, and shots to the other 

 peaks in the vicinity on the range can be laid down by the 

 station pointer with great precision, for the reason that when 

 the angles are small any slight error in placing the centre of the 

 station pjinter over the centre of the A has a minimum effect. 



The fact stated above should be borne in mind when plotting, 

 since it follows that a small error in the actual plotted position 

 of a A shows itself but very slightly when the line to be laid 

 off from that A makes only a small angle with the zero used. 

 Hence the desirability of choosing a zero for plotting which 

 makes a small angle with the line to be laid off from any 



If the triangulation has been calculated beforehand through- 

 out, and the lengths of all the different sides have been found, 

 it is more advantageous to begin plotting the main triangula- 

 tion by distances rather than by chords. The main stations 

 are thus got down in less time and with less trouble ; but these 

 are only a small proportion of the points to be plotted, and long 

 lines must be ruled between the stations as zeros for plotting 

 the other points by chords. In ruling these hues care must 

 be taken to draw them exactly through the centre of the 

 pricks denoting the stations ; but however carefully drawn, 

 there is liability to slight error in any line projected to a point 

 lying beyond the distance of the stations between which the 

 zero line is drawn. In plotting by distances, therefore, points 



