6825 HYDROGRAPHIC MANUAL PaGE 642 



station passes through extremely deep or very shoal water as compared to that traversed 

 to the other stations. 



The sound wave, in traveling an extremely short distance, may follow a direct 

 path, but over a long distance it is apt to be refracted and reflected a number of times, 

 thus making the travel time of the sound wave longer than it should be in proportion to 

 the horizontal distance. Similarly, the depth of the water traversed and the tempera- 

 ture gradient affect the travel time of the sound wave. It is apparent, therefore, 

 that an elapsed time under varying conditions is not exactly proportional to the hori- 

 zontal distance, although this latter is what is needed for plotting. Combinations of 

 distance arcs, where the above conditions vary extremely, will invariably plot errati- 

 cally if the same velocity of sound is used for all of them, and in such cases greater 

 weight should probably be given to the results from the nearer stations. 



If the same velocity of sound is used throughout an area, and the elapsed times 

 are uncorrected for the path of the sound wave, positions at a considerable distance 

 from the stations are likely to be incorrect even though the distances seem to be in 

 agreement. In such cases the stations are generally all in approximately the same 

 direction from the position and the distance arcs intersect at acute angles, the depths 

 are likely to be greater than average, and the temperature gradients will probably be 

 such as to cause excessive refraction and numerous reflections. The apparent position 

 will be in error, for the true position will be nearer to the stations by some amount 

 nearly proportional to their distances from it. Such displacements, although known to 

 exist, are difficidt to correct, because of the many variable factors that influence the 

 propagation of sound. 



The character of the bottom apparently affects the strength of a position very 

 little. Certain types of bottom materials may partly absorb the sound and reduce the 

 range of transmission, but the velocity of the reflected wave is probably not changed 

 enough to affect the elapsed times appreciably. 



6825. Inking Positions on the Boat Sheet 



Each accepted position (see 7635) should be pricked on the boat sheet and accentu- 

 ated with colored ink. The position number should be inked nearby for identification 

 (see 7681). The distance arcs may be inked or left in pencil, as desired. (See also 

 3251.) 



6826. Plotting With R.A.R. Distance Differences 



Ordinarily in R.A.R. the total elapsed time from each of two or more R.A.R. 

 stations is known and is used in plotting a position. However, if returns are received 

 from three or more R.A.R. stations appropriately located with reference to each other, 

 it is possible to plot the position by using the differences between the times to the 

 various stations, provided the velocity of sound is known, the total times themselves 

 being unlaiown. This is of practical value m R.A.R. where, for example, the record 

 of the bomb explosion is lost for any reason, as where»it cannot be distinguished on 

 the chronograph tape because of static. Such returns should not be rejected in an 

 R.A.R survey^ — they should be recorded in the Bomb Record for plotting at a later 

 date, if time and facilities do not permit then- being plotted at the time. 



There are several methods by which an R.A.R. position can be plotted graphi- 

 cally, using time differences, the choice depending on the base sheet used or on the 

 number of positions to be plotted. The position can also be found analytically by 



