3141 HYDROGRAPHIC MANUAL PaGE 198 



A system of 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 provides 

 a better development of long, narrow, steep-sided ledges or troughs. It is to be noted 

 in this connection that the axes of off -lying shoals of this nature are frequently approxi- 

 mately parallel to the trend of the coast. Likewise, off-lying sand and mud banks of 

 ridge form usually have their longer axes parallel to the coastline or parallel to the 

 prevailing coastal currents. 



In most modern hydrography, the depths are measured by echo sounding from a 

 survey vessel traveling at standard speed so that accurate compass courses can be 

 steered. Under such conditions systems of parallel lines are usually most convenient, 

 for approximately the same course will serve to follow alternate lines of the system. 

 For inshore hydrography, especially if by handlead, the sounding vessel is limited to 

 a slower speed, making the use of shore objects to control the position of the vessel on 

 line frequently advantageous or necessary. There are two general ways by which this 

 is done — (1) by the use of ranges and (2) by the use of distance angles (see 3143). 



'Ranges can be used to keep the sounding vessel on each of a system of parallel 

 lines, but to do this both the front and the rear range marks have to be changed for each 

 line. Coxswains with considerable experience in this type of work can be trained to 

 select objects for this purpose. The selection is facilitated by using a sextant to pick 

 up a front range mark toward which the sounding line leads. Immediately after the 

 sounding vessel has been placed on the outer end of the sounding line, and the position 

 fixed, the angle between the direction of the proposed line and a prominent control 

 station is scaled from the boat sheet, and with this angle set on the sextant, a front 

 range mark can be sought at the correct point on shore, the corresponding rear range 

 mark being farther inshore. (See 3453.) 



For a close systematic development of comparatively large ofi'shore shoal areas a 

 rear range mark can often be selected far enough inshore so that for all practical pur- 

 poses the sounding lines across the shoal are parallel to one another, 



A. ON OPEN COASTS 



For surveys along open coasts with gently sloping bottom, the principal system of 

 sounding lines should be either normal to or parallel to the general trend of the coast, 

 or a combination of the two directions may be used. The directions of the lines should 

 be chosen to give a complete and economic survey. Wliere the shore is a fairly straight 

 beach, the launch survey should generally consist of at least a few lines run close in and 

 parallel to the shore. At least enough parallel lines should be run to provide a sounded 

 zone for the launch to turn in when running the lines normal to the shore. The area 

 farther offshore where the slope of the bottom is gentle, may be surveyed by a system 

 of lines run in the most convenient arid economic direction. 



Where the slope of the bottom is irregular or steep, and in areas where pinnacle 

 rocks and steeply rising shoals or ridges may be expected, a system of lines normal to 

 the depth curves should be run. Where the submarine relief is in the form of a series 

 of narrow steep-sided ridges, or is indented by steep narrow troughs, the direction of 

 the system of lines must be such that the lines will cross these features at an angle. If 

 the axes of the features are parallel to the surrounding depth curves, a system of lines 

 normal to the depth curves will provide an adequate survey. But if the axes of the 

 features are perpendicular to the surrounding depth curves, as is the case with many 

 of the submarine valleys and canyons along the continental slope, a system of lines 



