Page 197 hydrography 3141 



such as ridges or submarine valleys, the system of lines adopted should cross the depth 

 curves at an angle of about 45°. 



The selection of the most appropriate system of lines for any given area requires an 

 understanding of the principles involved, which are discussed in 3141 to 3143 inclusive. 

 Three systems of sounding lines are in general use: (a) Parallel straight Imes, (6) 

 radiating lines, and (c) circular arcs. 



3141. Systems of Parallel Lines 



A system of evenly spaced parallel lines approximately normal to the depth curves 

 is most frequently used, especially near the open coast. Since the inshore depth curves 

 generally tend to parallel the coast, such a system of lines will also bo normal to the 

 coastline. The principal advantages of such a system are that the best delineation of 

 the depth curves is obtained with a minimum of sounding; the three-point fixes are 

 more easily obtained since fewer changes in objects are required; positions close inshore, 

 which cannot be fixed by sextant angles, can be reasonably well determined by course 

 and distance from the more strongly fixed positions farther offshore; such lines can be 

 extended into small bights and indentations in the shoreline and be reasonably well 

 controlled with a minimum of control stations ; and ranges and leading marks on shore 

 can be utilized to advantage in running the proposed lines. 



The principal disadvantages of such a system are encountered in the area immedi- 

 ately adjacent to the shore. It is difficult to extend such lines close enough to the 

 shore, and dangerous when heading inshore. Variations in speed when coming to a 

 stop or when starting a line frequently result in the displacement of some of the sound- 

 ings and, in turn, the depth curves. Such a system cannot be spaced adequately in 

 proportion to the depths, except by running numerous short splits, since it is obvious 

 that two parallel sounding lines run normal to the depth curves will be spaced correctly 

 for the depth at one place but will be too close together or too far apart elsewhere. 

 Also the prevailing winds and currents in many regions tend to parallel the coast and 

 would be normal to the vessel's course, thus adding to the difficulty of running the pro- 

 posed lines. 



To avoid many of the above difficulties a system of lines parallel to the shore can 

 be adopted for the survey of the adjacent waters, especially where the coastline has a 

 regular trend and an even, gradually sloping bottom. Such lines can be run closer to 

 shore since the sounding boat is running parallel to the danger line, instead of toward 

 or away from it. Longer lines can also be run, thus avoiding the numerous ends of 

 lines necessary in a system normal to the shoreline, and economy in operation can often 

 be attained by starting and ending the lines nearer to the anchorage or shore base. 

 Intervals between positions can be varied to take advantage of the locations of the 

 control stations to obtain strong three-point fixes. The lines closest to shore can be 

 run during abnormally high tides and a calm sea. 



A system of lines parallel to the coast is impracticable where the coastline has 

 many indentations or the shoreline is very irregular. Unless the control stations are 

 some distance inshore from the high-water line, the three-point fixes on the most in- 

 shore lines are likely to be weak since one angle is generally extremely large and the 

 other very small and they change very rapidly. If the control stations cannot be 

 located some distance inshore the number required to control the inshore lines is 

 excessive. 



