Page 267 hydrography 3465 



When the effect of such a current has not been foreseen m tune and the plotted 

 position of the vessel is found to be considerably off line, with a trend which will take it 

 farther yet from the line, it is sometimes advisable to make an S-shaped change in course 

 to put the survey vessel back on the proposed sounding line. This maneuver is the 

 same for a large survey ship as for a launch, except that it can be more quickly effected 

 in the case of a launch. Assuming that the position of the vessel has plotted con- 

 siderably to the left of the proposed line, as soon as this fact is known, the rudder is 

 put full right to make a change in course not exceeding 90° for the estimated time 

 necessary to maneuver back nearly to the line, when the rudder is put full left to resume 

 the original course, or rather a new course a few degrees to the right of the original one. 



The length of time to run on the turn can be deteiinined only by experience. In 

 a launch which turns quickly a run actually normal to the proposed line may be made. 

 In the case of a large survey ship the momentum of the ship prevents such a quick turn 

 and during the entire maneuver the ship is usually turning first to the right and then 

 to the left without having been steadied on any intermediate course. The times of 

 the various parts of the maneuver must be recorded accurately so that the actual 

 track of the vessel may be plotted on the smooth sheet. When three-pomt sextant 

 fixes are being used, a fix should be obtamed just before the maneuver is begun and 

 just after the vessel is steadied on the new course, if practicable. 



3465. Keeping a Lookout 



When surveying in areas where dangers to the survey vessel may be encountered, 

 a sharp lookout must be kept at all times. In handlead sounding from a launch an 

 adequate lookout can be kept by the leadsman, the coxswain, and the junior officer 

 when the last is not otherwise engaged. On a survey ship and a launch taking echo 

 soundings, a man whose sole duty it is to keep a lookout shall be stationed in the most 

 strategic position whenever the vessel is surveying in an area where unexpected shoals 

 or dangers may be encountered. In some regions, especially where the bottom is 

 coral, a lookout stationed aloft on a survey ship or at the greatest practicable elevation 

 on a launch will be able to detect shoaling by the appearance of the water and, in addi- 

 tion to warning the officer-in-charge of possible danger, will often sight shoals abeam 

 which would otherwise be missed on the regular system of lines. (See also 361 and 3623.) 



35. ADEQUACY OF HYDROGRAPHIC SURVEY 



In general, to be adequate the survey of any region should be sufficiently intense 

 and complete to determine the depths and the character of the bottom and to locate 

 all dangers and other features that should be charted for the guidance of the mariner. 

 The adequacy of a survey has a direct relation to the scale of the sheet on which it is 

 plotted — it is obvious that a completely adequate survey cannot be made on a scale too 

 small to plot the lines and soundmgs at the spacing required for a thorough develop- 

 ment of the area. In certain circumstances the Chief of Party has the authority to 

 increase the scale of a survey on his own initiative, and where he does not have this 

 authority he should not hesitate to recommend for the approval of the Washington 

 Office a larger scale of any area, when considered desirable (see 1214 and 123). 



351. A Basic Survey 



Hydrographic surveys may be classified as basic, revision, special, and reconnais- 

 sance. The project instructions will specify which is required. 



