6831 HYDROGEAPHIC MANUAL PaGE 646 



6831. Best Direction of Lines 



The direction of a. system of sounding lines is based principally on the bottom 

 relief but it is also selected so as to cover the area in a thorough and economic manner as 

 described in 3141A. In R.A.R. surveys there is an additional factor to consider, for 

 the system of sounding lines should not parallel the bisectrices between R.A.R. stations 

 if this can be avoided. Positions are comparatively difficult to obtain near a bisectrix 

 and impossible where the sounding line nearly coincides with a bisectrix. (See 6814 and 

 6823.) Therefore, insofar as practicable, R.A.R. sounding lines should be planned in a 

 direction more nearly normal to, than parallel to, the bisectrices. 



If the project instructions specify, or the bottom relief makes it desirable, that lines 

 be run in a certain direction, the R.A.R. stations should be established where the 

 bisectrices will cross the sounding lines at an angle greater than 45°, if practicable. 

 Most of the difficulties will thus be eliminated. 



6832. Splits and Development 



On large-scale surveys, where the sounding lines are closely spaced, it is difficult 

 to run splits and develop areas closely, using R.A.R. control only. Such lines have to 

 be run much more precisely than those of the general system, and the lapse of time 

 between a position and its plotting makes this difficult. 



Where a split has to be run between two closely spaced lines, a preliminary run of 

 sufficient length should be made to permit taking and plotting a trial position before 

 the line is reached. Best results are obtained if the preliminary run is made in the 

 same direction as the azimuth of the split, but it can be made at any angle with the line 

 if proper allowance is made for the turning radius of the ship when turning onto the 

 course. The course should be changed long enough before reaching the line so that the 

 ship will be on the correct heading when on line. 



Development lines are usually run normal to the direction of the general system of 

 lines, or at some angle for the best development of the feature. Generally such lines 

 must be closely spaced. Each line should be fixed by at least two R.A.R. positions, 

 and to provide for this it is occasionally necessary to run longer lines than would be 

 required just to cover the feature. For best control of a development line an R.A.R. 

 position should be obtained immediately after the ship is on the desired course. The 

 last position on a line, just before a change in course is made, may be plotted by dead 

 reckoning. 



684. Bombing Operations 



The final stage in bomb preparation, the ignition, and throwing the bomb over- 

 board, are operations which collectively are known as bombing in R.A.R. The final step 

 in preparing the bomb is made after a signal has been received from the chronograph 

 attendant or the bridge. At receipt of a second signal the bomb is ignited and thrown 

 overboard. After the personnel of a vessel have been engaged in R.A.R. for some time, 

 the procedure of bombing becomes routine and experienced personnel are able to per- 

 form their duties and obtain the data with a minimum of delay. 



The most important operation in bombing is the final preparation of the bomb, as 

 the detonator is inserted into the explosives in the bomb at this time. This operation 

 must be performed with the greatest care and attention, as it is the most dangerous 

 required in the handling of the explosives on board ship. 



