5. HYDROGRAPHY 



137 



tion of the sea bottom and submarine relief ; 

 and second, to reveal indications of shoals 

 or submerged dangers which are subse- 

 quently investigated for least depths (see 

 1-27 and 28). 



Generally a system of lines normal to the 

 depth curves provides the most convenient 

 and economic development of any area, but 

 it is often more advantageous to adopt some 

 other system for various reasons. The sys- 

 tem adopted for an open coast is not neces- 

 sarily suitable for bays and harbors. The 

 development of steep features such as sub- 

 marine ridges and valleys should be accom- 

 plished by a system of lines which cross the 

 depth curve at an angle of approximately 45 

 degrees. The selection of the most appro- 

 priate systems of lines for any given area is 

 governed by the type of control used, the 

 configuration of the area, and its location 

 with respect to an anchorage or base of op- 

 erations. Three systems of sounding lines 

 are in general use: (a) parallel straight 

 lines; (b) radiating lines; and (c) concen- 

 tric arcs (Fig. 54) . 



5-20 Systems of parallel straight lines. — 



A system of evenly spaced parallel lines nor- 

 mal to the depth curves and general trend 

 of the shore line is most frequently used 

 especially along an open coast. The principal 

 advantages of such a system are that the 

 best delineation of the depth curves is ob- 

 tained with a minimum of sounding lines; 

 three-point fixes are more easily obtained 

 since fewer changes in objects are required; 

 a position close inshore, which cannot be 

 fixed by sextant angles, can be reasonably 

 well determined by course and distance from 

 the last fixed position; and ranges on shore 

 can be used to advantage in running the 

 lines. The principal disadvantages of this 

 system are that it is difficult to extend the 

 lines close to the shore, and dangerous when 

 heading inshore; variations in speed at the 

 inshore ends may cause displacement of 

 soundings and lines of varying lengths must 

 be run appropriate to variation in depths. 

 When this system is adopted two or more 

 lines should be run parallel to the shore and 

 at high tide during calm weather. One line 



should be as close to the shore as circum- 

 stances permit, having due regard for the 

 safety of the launch. The second line should 

 be spaced about 50 meters offshore from the 

 first, and additional lines may be run as 

 necessary to provide a sounded zone for the 

 launch to turn in when running lines nor- 

 mal to the shore. 



When the coastline has an even trend and 

 a gradually sloping bottom, a system of lines 

 parallel to the shore may be used. In this 

 case the line spacing is gradually increased 

 as the depth increases. When such a sys- 

 tem is used, longer lines may be run and 

 there is less danger to the launch since the 

 inshore lines are run when sea conditions 

 are best. More frequent changes of fix are 

 required and the development of depth curves 

 is less accurate when this system is used. 

 A system of lines parallel to the coast is 

 impracticable where the shoreline is very 

 irregular or has many indentations. Un- 

 less the control stations are some distance 

 inshore from the high water line, fixes on 

 the inshore line are apt to be weak since 

 one angle is generally very large and the 

 other small and both change very rapidly. 



A system of straight parallel lines, whose 

 direction is at 45° to the depth curves, is 

 of advantage in certain areas. For the same 

 spacing between lines, such a system pro- 

 vides a better development of long, narrow, 

 steep-sided ridges or troughs. 



5-21 Systems of concentric arcs. — A sys- 

 tem of parallel arcs of circles may be used 

 to great advantage in some areas where 

 Shoran or Raydist equipment is used to con- 

 trol the survey. Such a system can be run 

 at any desired spacing by steering the sound- 

 ing vessel along a pre-selected arc of a circle 

 which is centered on the shore station. The 

 position of the vessel on the arc is fixed by its 

 intersection with the distance arc from a 

 second station. Course changes are required 

 at intervals depending on the speed of the 

 sounding vessel and the rate of curvature 

 of the arc. After a little practice the Shoran 

 or Raydist operator is able to keep very close 

 to the arc at all times. 



This efficient system has two important ad- 



