Page 709 the smooth sheet 7651 



nearest meridian. Where the line is short, changes in course may be plotted by their 

 angular differences, disregarding their true azimuths. 



7651. Determination of the Speed or the Log Factor 



For the plot of a section of dead reckoning the first step is the determination of the 

 speed of the ship or the log factor, if log distances are used. For this it is necessary to 

 select two well-fixed positions between which only minor changes in course have been 

 made. The actual distance over the ground between these two positions is compared 

 with either the log distance or the time run, in order to obtain a log factor or a speed 

 with which the dead reckoning is plotted. In hydrography controlled by R.A.R. 

 methods it is sometimes difficult to select accurately fixed positions. A good inter- 

 section of three distance arcs may usually be depended on as an accurate position, if 

 it is verified by the course and distance between it and an adjacent good position. 

 The speed or log factor should be determined from a comparatively long distance but 

 it must be remembered that speed and log factors differ with different directions of the 

 sounding line. A different speed or log factor must be determined after each major 

 change in direction. (See also 4826 and 4827.) 



7652. Dead Reckoning in Conjunction With R.A.R. 



Since there are still numerous uncertainties in the R.A.R. method of control, it is 

 frequently necessary to use a dead-reckoning plot in conjunction with the other data 

 to determine the most probable positions. There are two general cases where this is 

 necessary; first, in areas very close to the control, where for some reason or other the 

 R.A.R. data are inadequate; and second, on the outer ends of long lines distant from 

 the R.A.R. stations where few, if any, R.A.R. returns are obtained. The dead-reckon- 

 ing plot is made from the data in the Sounding Record supplemented and checked by 

 the R.A.R. abstracts. Where the dead reckoning is plotted by log distances, the use 

 of the R.A.R. abstracts is necessary because only on this form are the log intervals 

 determined. (See also 7634.) 



Dead-reckoning plots are not necessary in connection with all R.A.R. sm*veys. 

 In areas where the positions are controlled by three R.A.R. distances which plot con- 

 sistently with good intersections- the dead-reckoning plots are unnecessary. On the 

 other hand, where the R.A.R. retm-ns are not consistently and accurately received for 

 any reason whatsoever, the control data should always be supplemented by the dead- 

 reckoning plot; for example, dead-reckoning plots should always be used in plotting the 

 outer ends of offshore sounding lines. It is good training for an inexperienced person 

 to plot the dead reckoning of all of the lines controlled by R.A.R. The dead-reckoning 

 plots may also be advantageously used by the supervising officer in verifying the work 

 of a new employee. 



7653. Plotting a Dead-Reckoning Line 



There are several methods by which dead reckoning may be plotted, the choice 

 between which depends on the amount which is to be done. Parallel rulers may be 

 used. With them the successive courses may be plotted with reference to a meridian 

 line or by the small angular differences between the courses. A circular no-arm 

 protractor may be used. This is considered a better method than the use of the 

 parallel rulers. In this method each successive course is plotted in azimuth relative 

 to the previous course. 



