Page 273 hydrography 355 



The general system of sounding lines is intended to cover the area systematically 

 and determine the general depths. The spacing of the lines and soundings should be 

 close enough so that at least an indication of every shoal or danger is given. Extensive 

 experience is not required in the mechanical operation of running systematic sounding 

 lines, but it must not be assumed that the general system of lines will adequately 

 survey an area. The development of the indications furnished by the general system 

 is always the most important part of the work and frequently the most extensive. 



The extent and kind of development needed in a region is often disclosed by a 

 study of the adjacent land forms. For example, in certain water areas along the New 

 England Coast, sunken boulders, corresponding to those that dot the fields along the 

 shore, are found in great numbers, while in southeast Alaska the sharp mountain 

 peaks, that are so noticeable while cruising along the coast, are duplicated under 

 water as submerged pinnacle rocks, often rising from great depths to within a few feet 

 of the surface. On the South Atlantic Coast of the United States the low even sandy 

 coasts give assurance that dangers, such as rocks and reefs, do not exist, but here it 

 will be found that, corresponding to the constant movement of the sand dunes along 

 the shore under the effect of the wind, there are frequent changes in the bottom due 

 to the action of waves and currents. 



A mountain range, or ridge, gradually decreasing m elevation to a rocky point at 

 the shore, is likely to be extended into the water in the form of a submarine ridge or a 

 series of shoals. Near the axis of a chain of islands, submerged features of a similar 

 nature are likely to exist. 



For the adequate development of these indications and such features as channels, 

 anchorages, and areas inside the 10-fathom curve, modifications of the methods used in 

 the systematic survey are often required to secure a complete and economic develop- 

 ment of the features. 



Development must be distinguished from examination. Development is to ensure 

 that there are no dangers in the area, to provide for the accurate delineation of the depth 

 curves, and to locate the positions of existing dangers. Development is not expected 

 to furnish the least depths on dangers and shoals; these are obtained by examination 

 (see 366). 



The extent of the development will vary from a maximum on shoal areas in impor- 

 tant locations and in channels and anchorages having depths near the draft of vessels 

 to be accommodated, to a minimum on extensive shoal flats, over which navigation 

 is not expected, and in clear areas of much greater depth than required for navigation. 

 (See also 367 and 368.) 



Shallow channels and navigable waters that are likely to be used by shallow-draft 

 vessels or motorboats must likewise be developed. 



In regions where changes are continually taking place, development ordinarily 

 need not be so detailed as in regions of little change. 



The development of all shoal indications is one of the most essential details of 

 hydrographic surveying. In order that all soundings indicative of possible shoals may 

 be noted, close cooperation between the hydrographer, the recorder, and the leadsman 

 or fathometer attendant is essential. Shoals should be developed by a closely spaced 

 system of crosslines to determine the positions of the least depths at which a marker 

 buoy may often be anchored as a guide in the subsequent examination (see 3665). 



Where irregularity in depth is disclosed during the survey or where it may be 

 expected from the rocky nature or formation of the coast, the slightest indication of a 

 danger or shoal must be thoroughly developed. The narrow tongue of a shoal either 



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