3152 HYDROGRAPHIC MANUAL PaGE 202 



Graph AA in figure 55 is applicable to coastal areas of monotonously flat sand or mud bottom, 

 where rocks are unknown and sand banks and shoals are not to be expected, such as the Gulf Coast 

 of the United States. 



Graph BB should be used for coastal areas where the bottom is predominantly sand, and rocky 

 formations are unknown but banks may be found, such as the East Coast of the United States south 

 of New York. 



Graph CC is applicable to areas where dangers and rocky formations are likely to exist, such as 

 off the West Coast of the United States and Alaska. 



Graph DD should be used in areas where there are frequent dangers to navigation and steep rocky 

 formations rising from the bottom, or where these may be expected from a study of the adjacent terrain, 

 such as in the waters off the coast of Maine, and in some of the inland waters of Alaska. » 



It will be noted that all of the above-mentioned graphs coincide at a spacing of 5 miles, which is 

 the maximum to be tolerated for the deeper depths of any survey which is part of a project along 

 a coast. 



In a search for an oceanic danger or a steep submarine mountain rising from oceanic depths, 

 the lines must be at closer intervals in the shoaler depths, but may be at intervals of as much as 10 

 miles in depths of 3,000 fathoms. For any depth, the maximum required spacing is given by 

 graph EE. 



Since the most economic survey of a coastal area is usually by a system of parallel 

 lines normal to the depth curves, it is obvious that a spacing which is adequate at the 

 inshore ends of the lines will be closer than required by the same formula for the offshore 

 ends of the lines. To attain economy in surveying where such a system of lines is used, 

 the interval between lines must be changed correspondingly with the changes in depth. 

 The most satisfactory method of doing this economically is to fix the intervals between 

 lines at multiples of one another. 



If an area is to be surveyed on one sheet from one sounding vessel, in most cases 

 it will be more economic to run the most widely spaced lines first, successively splitting 

 the lines until the required spacing is attained. For example, suppose the project 

 instructions specify the following spacing — ^200 meters at 5 fathoms, 400 meters at 12 

 fathoms, and 800 meters at 30 fathoms. (It is to be noted that these are the maximum 

 allowable spacings at the respective depths, and that a proportional spacing is required 

 at other depths — for example, at 21 fathoms the spacing should be not more than 600 

 meters.) To meet the requirements most economically with a system of parallel lines 

 normal to the depth curves, lines 800 meters apart should be run first, from the shore 

 to the offshore limits of the project. These should then be split from the shore to 

 depths of 30 fathoms (necessary because the allowable spacing of 800 fathoms only 

 begins in depths of 30 fathoms), and then split again from the shore to depths of 12 

 fathoms, using as a guide the depths obtained on lines already run. By this method, 

 the positions of the depth curves will be known approximately from the 800-meter 

 system and the hydrographer will know just how far oft'shore to run the splits. If the 

 reverse procedure is followed, the inshore lines will almost invariably be run too far 

 offshore in order to ensure that the required depth curve has been reached and that 

 there is no shoaling outside of this which requires the closer spacing. Furthermore, 

 the hydrographer runs the risk of having to run additional splits if an area of shoaler 

 water is subsequently disclosed offshore from the outer ends of the closely spaced 

 inshore lines. 



3152. Sounding-Line Spacing for Inside Waters 



The general spacing for inside waters is 100 meters in bays, passages, channels, 

 rivers, etc. This spacing may be increased to 200 meters in areas of considerable 

 extent where uniform bottom without dangers is anticipated. 



