3372 HYDROGRAPHIC MANUAL PaGE 234 



or depression angles, the remainder being controlled by dead reckoning. Positions 

 should be spaced approximately 6 or 7 minutes apart, the logs and time being recorded 

 in addition to any other data. If a sono-radio buoy is used, a distance to it can be 

 obtained at each position and supplemented by other dead-reckoning data. 



(3) To supplement astronomic observations. In this case the lines are long, with 

 no more than minor changes in course between the astronomic data. Such dead reckon- 

 ing is similar to that used by every mariner, except that it is carried out with greater 

 precision. 



3372. Importance of Accuracy 



The accuracy and hence the value of dead reckoning depends on the precision 

 with which the various elements entering into it are known and are observed. The 

 true course should be known within 1° and, if a gyrocompass is not used, the deviations 

 of the magnetic compass should be from a recent ship swing; and, if a gyro pUot is 

 not used, an experienced helmsman must be selected who can be trusted to maintain 

 an average course within a fraction of a degree. The officer on watch should check the 

 course frequently. 



Currents are not observed, but the effect of the current at the beginning and end 

 of the line may be deduced from the fixed positions, and its estimated effect, as well 

 as the estimated leeway, must be applied throughout the dead reckoning. The possi- 

 bility of a change in current during the run must not be disregarded. Leeway is 

 probably the most uncertain of all corrections. The log factor should be known 

 within two decimals. 



The probable error in each of these elements is small in comparison with the 

 length of the lines, but to attain the required accuracy all must be observed with more 

 than average care and precision. 



3373. The Dead-Reckoning Course 



If the vessel is not equipped with a gyrocompass, it is essential to keep the magnetic 

 compass adjusted so that courses steered by it will be nearly magnetic. Unless the 

 deviation card is based on a recent ship swing, several of the values used should be 

 verified either before or during the running of the dead-reckoning line. 



The courses on long dead-reckoning lines can be set by reference to the standard 

 compass, but where comparatively short loops are involved it is advantageous to know 

 the deviations of the steering compass accurately enough so that all courses may be 

 set directly by it. 



It is important that the course be accurately steered and an experienced helmsman 

 should be selected to steer all dead-reckoning lines, unless the ship is provided with 

 a course recorder, from wdiicli the average courses steered can be subsequently deter- 

 mined. 



The course can be more easily and accurately steered in a smooth sea and, since 

 dead-reckoning loops are usually only a small percentage of the entire survey, they 

 should be undertaken only during the most favorable weather. In a rough sea it is 

 very difficult to maintain the ship on the correct heading on certain courses, for example, 

 when there is a following sea. 



Large changes in course should be kept at a minimum in dead reckoning and all 

 proposed lines should be planned with this in mind. Changes in course of 15° or less 

 can be assumed to be effective at the middle of the ship's swing aiid a record of the time 

 and log reading at this point is sufficient. When the change is more than 15'^ the time 



