62 HYDROGRAPHICAL SURVEYING [chap. ii. 



Triangu- AH surveys are, however, alike in this respect. They are, 



* ^°°' as it were, built up on a framework of triangles of some kind, 



the corners of which are the main " points " of the chart, and 



to obtain tliis framework is always the first thing to do, and 



how to set about it the first thing to consider. 



The construction of this " triangulation," as it is termed, is 

 of various kinds ; ranging, from the rough triangles obtained 

 in a running survey, where the side is obtained by the distance 

 it is supposed the ship has moved, and the angles are sextant 

 angles, taken on board from a by no means stationary position, 

 to the almost exactly formed triangles of a detailed survey, 

 when carefully levelled theodolites observe the foundation of 

 a regular trigonometrical network, which covers the whole 

 portion to be mapped. 



The term " triangulation " would seem to infer that this 

 system of triangles would be always apparent ; but in surveys 

 irregularly plotted, and when working on a sheet previously 

 graduated, it will seem that there is no triangulation, and in 

 the strict sense of the word there is none, but the framework 

 of the chart is still built up on the system of triangles, and 

 it is difficult to find any other name for the process. 



For the present we will speak only of the second and third 

 kinds of surveys, leaving Sketch Surveys to be described 

 separately. 



The system employed in Ordinary Surveys and Detailed 

 Surveys is the same, and they really only differ in the scale 

 of the chart, and the amount of time that is spent on them, 

 especially with regard to closeness of soundings. 



In a detailed survey, time must be subservient to the 

 necessity for exactness, and for exploring every foot of the 

 ground. 



In an ordinary survey, judgment has to be exercised as to 

 how far we must be satisfied with what we can get for triangu- 

 lation, and how much time we can spend on details. 



It is by no means necessary in an ordinary survey to observe 

 the angles at each corner of the triangles. The happy fact that 

 the sum of the three angles is 180° enables us to manage 

 whenever we have two of them, though it is, of course, more 

 satisfactory to actually observe all three for the more important 

 triangles. 



