Page 259 hydrogeaphy 344 



in the docks and slips. The Hnes and soundings should be referenced to the wharves and 

 piers by measured distances using the method described in 3344, or a substitute there- 

 for which will give comparably accurate results. Extra precaution should be taken 

 that all soundings are vertical, are measured by pole or an accurate handlead, that the 

 tide is accurately known, and that the position of each sounding is accurately known 

 with reference to the wharf. The sounding lines and the soundings along the lines 

 shall be as close together as practicable for this type of survey. 



In addition to any other lines which may be run, soundings shall be taken from 

 the edges and faces of the piers and along the line likely to be occupied by the keels 

 of vessels berthing there. These data in the vicinity of wharves and docks should be 

 shown on subplans m accordance with 7751, wherever the scale of the regular survey 

 of the area is too small to show the data adequately. 



344. The Operation of Position Fixing 



Although the depth of the water at any specific place may be measured with con- 

 siderable accuracy, this knowledge is valueless unless the geographic position of the 

 measured depth is known, or its position is referenced to adjacent land features. The 

 operation of determmmg the horizontal position of a soundmg or depth measurement 

 is known as position fixing. This is a procedure which must be repeated along the 

 sounding line at sufficiently short intervals, depending on the scale and the spacing 

 of the soundings and the lines. 



When practicable, fixed positions should be obtained at regular intervals and each 

 position should coincide with a sounding to facilitate spacing and locating the inter- 

 mediate soundings. This is not an essential requirement, however, and in certain 

 kinds of hydrography is disregarded. Irregular intervals may be necessary to comply 

 with other requirements, and the difficulty of obtaining a position on a sounding may 

 be greater than the inconvenience that is caused in the subsequent plotting of the 

 soundings. 



The actual operations of position fixing by various methods are described in 

 section 33 and the frequency with which positions shoidd be obtained for each type of 

 control is specified in 3313 and 6812. 



3441. A Sextant Fix 



The sequence of events that take place, and the orders and oral reports which are 

 given may perhaps be better understood by the following consecutive relation. The 

 relation is only continued through all of the events in connection with one three-point 

 fix, but from this an understanding of the continuous operations which occur can be 

 had. The time of the position is 10:00:00 and all of the data obtained are recorded in 

 the Sounding Record as of this time; other times given below are merely to illustrate 

 the approximate timing of the various activities in connection with obtaining that fix 

 and sounding. It is assumed that a launch party is surveying with handlead and using 

 three-point fix control. The officer-in-charge observes the left angle and there is a 

 right angleman in addition to the recorder. 



It must be understood that, in this case, all of the personnel are comparatively 

 close to one another and that all orders, directions, and reports are made orally and can 

 be heard by all concerned. The recorder, leadsman, and coxswain repeat all orders 

 and the recorder lepeats all data as he records them. In some cases signal bells may 



