80 HYDROGRAPHICAL SURVEYING [chap. iv. 



skill of the chief of the survey is shown in overcoming these 

 difficulties in the readiest and best method, and these are the 

 circumstances on which we can give the fewest hints. Such 

 as we do mention will be found in the next chapter on Plotting. 

 In the present one we shall confine ourselves to regular tri- 

 angulations. 

 Triangu- It is necessary to make a preliminary run over the ground 

 Coast Sur- ^^ ^^^^ Suitable positions for main and secondary stations on 

 vey. prominent headlands, islands, and summits not too far back 



from the coast. If no former survey exists, a rough plot 

 should be made by compass and patent log. A scheme must 

 now be formed for the main triangulation, with the object of 

 enclosing the whole survey in as few triangles as possible, 

 regard being paid to the limit of vision of each station due to 

 its height, to the existing meteorological conditions, to the 

 limitation imposed by higher land intervening, and to its 

 accessibility. 



The triangles decided upon should be well conditioned, taking 

 care not to introduce an angle of less than 30 degrees. A 

 smaller angle is permissible, however, when the two longer sides 

 of such a triangle are of nearly equal length, and when in the 

 calculation that will follow one of these sides shall be derived 

 from the other longer side and not from the short side. 



In open country the selection of stations is a comparatively 

 easy matter. In country densely wooded the time occupied by 

 a triangulation is mainly governed by the judicious selection 

 of stations quickly reached, sufficiently elevated to command 

 distant views, and situated on summits capable of being readily 

 cleared of trees in the desired direction. An all-round view is, 

 of course, desirable, but not always attainable. 



The object of secondary stations is to break up the large 

 primary triangles into smaller ones, dividing up the distances 

 between the primary stations into suitable lengths. They are 

 selected with a view to greater accessibility than the latter, 

 and should therefore usually be near the coast and at moderate 

 elevation. 



Upon shots from these secondary stations will depend the 

 position of the greater number of the coast-line marks presently 

 to be erected and fixed as the detailed survey of each section 

 of the coast is taken in hand. The nature of the base to be 



