Page 199 hydrography 3142 



whose direction is approximately 45° to both the depth curves and the axes should be 

 selected. 



On the continental slope itself, where the gradient is comparatively steep, a 

 system of lines perpendicular to the depth curves gives the best and most economic 

 development. 



An extensive shoal area with no pronounced axis may be adequately surveyed 

 by a system of parallel lines run in any convenient direction. If it has a definite axis, 

 the best development will be obtained by a system of lines run at an angle of 45° to 

 the axis, supplemented by several closely spaced lines along the axis. A shoal of 

 circular shape whose least depth may be expected near the center is best developed by a 

 system of radiating lines converging near the shoalest spot (see 3142). 



B. IN PROTECTED WATERS 



In protected waters of large area, the same general considerations apply; in general 

 a system of lines approximately normal to the depth curves will provide the most 

 adequate development. A few lines close inshore should always be run parallel to the 

 shoreline except where it is so indented that this is impracticable. The lines should 

 be run during periods of high tide and in calm weather. 



In small baj^s, harbors, and similar indentations of the coast, the dhections of the 

 lines will depend almost enthely on the configuration of the feature. They should be 

 laid out so as to give a complete and economic development. 



Extensive flats which bare at low water generally do not requhe closely spaced 

 lines. A few widely spaced lines should be run over such areas to define the limits of any 

 navigable channels therein which need to be more thoroughh^ sounded, and the low- 

 water line should be located by a zigzag line of soundings run at high tide. The outer 

 edge of the flat should be located by a system of parallel lines which will determine the 

 line of demarcation between shoal and deep water. 



Tests have prove'd conclusively that vertical handlead soundings cannot be obtained 

 in strong currents, except when proceeding in the direction of the current. When 

 channels or streams are sounded with the handlead, the entire development must consist 

 of a series of lines parallel to the axis, the lines being run at slow speed in the direction 

 of the current (see 3464) . 



Echo soundings are not affected by the current, and when they are used, a channel or 

 river should be sounded first by a system of lines running across the axis of the channel, 

 either normal to or diagonal to it. Diagonal lines should be used in narrow chan- 

 nels and where the slope is abrupt at the edge of the channel. The lines should be run 

 at such an angle that the change in depth will be gradual enough to permit taking a 

 sufficient number of soundings to locate the edges of the channel and the desired depth 

 curves. All variations in sounding speed must be noted in the Sounding Record with a 

 sufficient number of positions taken so that there will be no doubt as to the correct 

 positions of the depth curves. The system of crosslines will serve to locate the channel 

 which must then be thorouglily developed by a system of lines parallel with its axis. 



3142. Systems of Radiating Lines 



Off points where there is a marked change in the trend of the shoreline, in small 

 bays, and around off-lying islets, radiating lines generally provide the most adequate 

 development. It is to be noted that such lines can frequently be run around points 

 by using a rear range mark selected so that the convergence of the lines near the shore 

 provides the most suitable spacing proportional to the changing depths. 



