184 



HYDROGRAPHICAL SURVEYING [chap. vi. 



Bearings 

 and 



Vertical 

 Angles. 



Colonial Trigonometrical Survey when tlie latter was completed 

 several years afterwards. 



An instance is thus afforded of the remarkable accuracy 

 that may be attained by a floating beacon survey, notwith- 

 standing the fact of landing being impracticable between the 

 terminal points. 

 Sketch In the case of an island culminating in a high, well-defined 



Compass^ summit, visible from all directions, a useful and accurate method 

 is to steam round it at a suiBcient distance to obtain a true 

 horizon, stopping to make as many stations as may be desirable, 

 and fixing by compass bearing of the summit and its vertical angle. 



The height is roughly obtained by shooting in its summit 

 from two positions on a patent log base whilst approaching it. 



With this approximate height and Lecky's Vertical Danger 

 Angle Tables, each station may be plotted on its bearing of 

 the summit. 



From these stations the island is shot in by angles between 

 its tangents and its summit, and angles to any other natural 

 features, plotting the work as we go on any convenient scale, 

 which must be considered only as provisional. 



On completing the circuit of the island, the true scale is 

 found by measuring the total distance in inches on the plotting- 

 sheet, from the first to last stations, and dividing it by the 

 distance between them in miles as shown by patent log. 



The final height of the summit bears to the rough height 

 used in plotting the direct proportion of the provisional scale 

 to the true scale. 



This method may be utilised for the sketch survey of a 

 coast where there are well-defined peaks of sufficient height 

 at convenient intervals, and would be superior to an ordinary 

 running survey. 



From positions of the ship fixed by bearings and elevations 

 of one peak, another farther along the coast is shot in and its 

 height determined ; this second peak is then used in its turn 

 to fix a third, and so on. 



The smaller the vertical angle, the more liability there is to 

 error ; but a glance at Lecky's tables will show what effect an 

 error of, say, 1' in altitude will produce for any given height 

 and distance, and the hmit of distance must depend upon this 

 consideration. 



