GO 



HYDROGRAPHICAL SURVEYING [chap. ii. 



DifiFerent 

 Kinds of 

 Surveys. 



Sketch 

 Sxirveys. 



Ordinary 

 Surveys. 



Detailed 

 Surveys. 



If circumstances permit the use of floating beacons, the 

 accuracy of such work is much increased. 



In the first place, Marine Surveys may be divided into three 

 heads : 



1st. Prehminary or Sketch Surveys. 



2nd. Surveys for the ordinary purposes of navigation. 



3rd. Detailed Surveys. 



The boundaries between these are by no means strongly 

 marked, although each differs considerably from the other, and 

 a finished sheet as sent home is not unfrequently a combination 

 of all three, comprising pieces of work done after very different 

 fashions, according to needs and circumstances. 



A preliminary survey does not pretend to accuracy. The 

 time exf)ended on it, and the means used, cannot ensure it, 

 and it only represents what our second name for it indicates, 

 a sketch. A sketch survey will be founded on a base of some 

 Idnd, but this %vill generally be rough, and in some instances, 

 as in many running surveys, will depend solely upon the 

 speed of the ship as far as it can be ascertained by patent log ; 

 so that the whole affair from beginning to end is only a rough 

 approximation. 



The necessity for sketch surveys may be said to be getting 

 less and less every year. Most parts of the world have their 

 coasts mapped at any rate as far as this ; but there still remain 

 portions of our globe of which the coast-lines are not marked 

 at all, or are extremely hazily delineated, and to these any 

 sketch survey will be an improvement. 



The second head comprises the majority of charts now pub- 

 lished, and many of those in course of construction in the 

 present day, i.e., they are constructed on such a scale, and 

 with such limitations of time, etc., as to make it impossible 

 either to show small details of land or sea, or to be perfectly 

 certain that small inequalities of the bottom, or detached rocks, 

 may not exist, unmarked. Everything, however, shown in 

 such a survey should be correct, and it is only in its omissions 

 that it should be imperfect. 



A detailed survey is accurately constructed from the com- 

 mencement, on a scale large enough to admit of close sounding, 

 and time is given up to working out all minutiae. 



Detailed surveys are mostly confined to the more civilised 



