Page 237 hydrography 3377 



error or less accurate than the dead reckoning and there should be no hesitancy in 

 rejecting them when they are found to be so. 



3377. The Dead-Reckoning Abstract 



A dead-reckoning abstract on Form 722, R.A.R. and Dead-Reckoning Abstract, 

 should be kept as the dead-reckoning line is run. On this abstract all control data 

 should be entered which are to be used in plotting and adjusting the dead-reckoning line 

 and also the courses and the distances made good insofar as can be estimated. Each 

 officer on the bridge has his own duties in connection with obtamiiig, recording, and 

 plotting these data. If there is a considerable lapse of time between the running of 

 dead-reckoning loops, the plotting and adjustment of a loop may be performed at any 

 convenient time after the run has been made. If, however, one dead-reckoning loop 

 is run immediately following another, it is essential that the plotting and adjusting be 

 kept up to date. Usually the dead reckoning can be plotted as it proceeds and a 

 completed loop can be adjusted while the next loop is being run. 



3378. Adjustment of Dead Reckoning 



The dead reckoning should be corrected for all known factors affecting course 

 and distance before it is plotted, so that it represents the actual track of the vessel as 

 accurately as possible before adjustment to the terminal position. The amomit and 

 direction by which the terminal position plotted by the dead reckoning fails to check 

 the terminal position determined by fix is the dead-reckoning closure. This dead- 

 reckoning closure is usually first proportioned by time throughout the entire run, in the 

 same way that a traverse closure in ground surveying is proportioned tlu^oughout a 

 traverse according to distance. 



The above is usually only the first step in the adjustment of a dead-reckoning loop; 

 there are other control data which must be taken into account before the adjustment 

 can be considered complete. The courses as determined by the several fixed positions 

 at the beginning of the line and by the several fixed positions at the end of the line 

 must be given weight at the beginning and end of the dead-reckoning loop. The 

 effect of this is that the first and the last portions of the dead reckoning must frequently 

 be plotted as flat curves rather than straight lines. Other intermediate data such as 

 single bomb distances, loci of single angles, and bearings, are plotted and each is analyzed 

 with reference to the preliminary adjusted dead reckoning. Those partial data which 

 appear reasonably correct and probably stronger than the dead reckoning itself are 

 accepted and the dead reckoning is readjusted to make it conform to them. (See also 

 764 and fig. 161.) 



3379. Precise Dead Reckoning 



Precise Dead Reckoning was the name given to a type of accurate dead reckoning used before 

 the advent of echo sounding in depths where the survey ship could be anchored. The ship ran at 

 reduced speed in order to obtain the soundings. The most notable characteristic of this type of dead 

 reckoning was that the ship was anchored for current observations at the beginning and end of each 

 sounding line and at intervals of about 2 hours on the line. This method was described in Special 

 Publication No. 73, Precise Dead Reckoning in Offshore Soundings. 



Still later, additional accuracy was gained by anchoring lines of buoys perpendicular to the 

 coast, spacing the buoys about 10 miles apart along the lines and the parallel lines of buoys about 

 10 miles from each other. The buoys were located by full speed double runs to obtain accurate log 

 distances and dead reckoning. The sounding lines were then run at reduced speed parallel to the 



