Page 705 the smooth sheet . 7634 



7634. Adjustment of the Dead-Reckoning Plot 



After the distance arcs and the other position data have been plotted in pencil and 

 the dead reckoning for that part of the hydrography has been plotted on tracing paper 

 (see 765), a study is made of the dead reckoning to fit it best to the distance arcs and 

 the other position data. In areas comparatively close to the R.A.R. stations where 

 most of the bomb distances are reliable, this study is a comparatively simple matter. 

 But where the control is inadequate or unreliable, as is often the case in areas distant 

 from the control stations, the adjustment may be difficult even for one with extensive 

 experience in R.A.R. methods. Continued and determined study in such cases fre- 

 quently yields results seemingly impossible of attainment when the first attempt at 

 adjustment is made. 



The two positions which were used to determine the speed or log factor must 

 not be given too much weight since it is always possible that one of these may be in 

 error in spite of the fact that they were originally selected as being probably correct. 

 It is to be noted that because these were used in determining the speed or log factor 

 the dead-reckoning plot will fit them regardless of their actual accuracy of position. 



Occasionally a series of three R.A.R. distances is obtained from one station 

 which is relatively close to the sounding line. If the true azimuth of the sounding line is 

 accurately known, the dead reckoning may be fitted to these three arcs in the same 

 manner it would be fitted to bow, beam, and quarter bearings. 



Where the dead-reckoning plot cannot be fitted satisfactorily to the position 

 data, this is occasionally due to the use of an erroneous speed or log factor, in which 

 case the dead reckoning must be replotted. 



The outer ends of long ofl'shore sounding lines are perhaps the most difficult to 

 .adjust. These are generally far distant from the control stations where the inter- 

 sections of the distance arcs are weak at best and where, because of the extreme dis- 

 tances, many of the bomb returns are unreliable. Wliere the line is in extremely deep 

 water, as it generally is, there is an added uncertainty due to the incomplete loiowledge 

 of the path of the sound wave in such areas, and consequently the apparent velocity 

 needed for plotting. Some of these dead-reckoning runs are 6 to 8 hours or more long, 

 during which period a considerable change in the leeway may be caused by wind or 

 weather, and a change in the direction of the current, if tidal, may be expected. There 

 is never sufficient knowledge of the current on such an offshore line. 



Such offshore sounding loops must be adjusted to the last two or three positions 

 on the offshore run that are believed to be reliable and the first two or three similarly 

 reliable positions on the inshore run. The dead-reckoning plot is adjusted approxi- 

 mately for azimuth to the reliable positions at the beginning of the line. The closing 

 error at the end of the line is adjusted graphically, proportional to time, in the man- 

 ner that a closed traverse is adjusted. This first adjustment is made on tracing paper. 

 An additional adjustment is usually required to fit it to the R.A.R. data in order to 

 give the distance arcs their appropriate weights. (See 3376 and 3378.) 



If there are crosslines in such an area as has been discussed in the preceding para- 

 graph, a study of the boat sheet should be made to determine how well the respective 

 lines have been controlled. The best-controlled line should be plotted ffi'st and the 

 successive lines plotted in the order of the strength of their control. On each line the 

 soundings at the crossings should be tentatively plotted and used as additional control 

 data, the positions of the more poorly controlled lines being influenced by those that 

 are better controlled. 



465382—44 46 



