5. HYDROGRAPHY 



141 



to permit an accurate portrayal of the bot- 

 tom configuration. 



5-27 Line spacing on open coasts. — The 



spacing of sounding lines on open coasts 

 depends on the depth of water, type of bot- 

 tom, and scale of the survey. In areas such 

 as the Gulf of Mexico and parts of the Atlan- 

 tic coast where the bottom is composed mostly 

 of sand or mud and depths increase slowly, 

 sounding lines can usually be spaced at twice 

 the interval which can be safely used in 

 areas of irregular bottom. In areas of smooth 

 bottom, along an open coast a line spacing of 

 200 meters in depths less than 11 fathoms, 

 400 meters in depths or 11 to 20 fathoms, and 

 800 meters in greater depths will be adequate 

 However, at entrances to harbors, in areas 

 adjacent to spits or rocky points, and in areas 

 where major changes in bottom contours are 

 found, the spacing shall be reduced one-half. 

 In areas of irregular bottom on open coasts 

 the line spacing shall not exceed: 100 meters 

 in depths less than 20 fathoms around rocky 

 points and spits and in entrances to channels ; 

 200 meters in all other areas where the 

 depth is less than 20 fathoms ; 400 meters in 

 depths of 20 to 30 fathoms and 800 meters 

 in depths of 30 to 50 fathoms. 



5-28 Line spacing for offshore surveys. — 



Hydrographic surveys in offshore areas may 

 be plotted on scales as large as 1:40,000 

 where they join inshore surveys, or may be 

 plotted on a scale as small as 1:500,000 in 

 ocean areas of great depth. Regardless of 

 the type of bottom the line spacing shall not 

 exceed: 1,600 meters (1 mile) in depths of 

 50 to 100 fathoms; 3,200 meters (2 miles) 

 in depths of 100 to 500 fathoms; and 5 miles 

 in greater depths. The nature of the bottom, 

 the importance of the area, and the scale of 

 the survey, will be considered when project 

 instructions are written. In areas of rocky 

 or broken bottom, closer spacing will be re- 

 quired and different break-points may be 

 specified. The spacings specified above are 

 maximums which must not be exceeded. 



It is obvious that such wide spacing is not 

 adequate to permit detailed contouring of 

 the bottom. The surveyor must study the 



the soundings to detect indications of sub- 

 merged features which must be surveyed in 

 greater detail. The general line spacing 

 prescribed for the area or depth shall be 

 reduced as necessary to determine the con- 

 figuration of submerged mountains, valleys, 

 trenches, and canyons and to determine the 

 limits of escarpments. Smaller features such 

 as mounds, seaknolls, and depressions should 

 be developed. Any breaks in slopes along con- 

 tinental or island shelves should be defined. 



5-29 Sounding interval. — Although 

 graphic recording echo sounders are used to 

 obtain a continuous profile, it is the usual 

 practice to record soundings at fixed inter- 

 vals as the lines are run. The hydrographer 

 shall select a time interval for recording 

 soundings appropriate to the scale of the sur- 

 vey, the depth of water, the configuration 

 of the bottom, and the speed of the sounding 

 vessel (see 1-33). Where the depths are 

 uniform or the slope is uniform, the maxi- 

 mum interval between recorded soundings 

 shall be that which will permit plotting all 

 soundings on the smooth sheet without con- 

 gestion (see 6-59). 



In areas of irregular submarine relief, too 

 much importance must not be attached to the 

 maintenance of a uniform interval. A uni- 

 form interval facilitates the plotting of the 

 smooth sheet, but it is more important that 

 the recorded soundings give a true repre- 

 sentation of the irregularities of the profile. 

 In such areas the least depths must be re- 

 corded and between them the deepest 

 depths with as many intermediate soundings 

 as are required to define the profile ac- 

 curately. At least as many soundings shall 

 be recorded as can be shown on the smooth 

 sheet. In order that a sounding may be plot- 

 ted in its correct position, it is essential that 

 the exact time of observation be recorded. 

 Times of positions and regular interval 

 soundings shall be recorded from the sound- 

 ing clock. The time of an extra or odd inter- 

 val sounding shall be scaled from the fatho- 

 gram and shown as a fraction of the regular 

 sounding interval (see 5-95). 



The following rules will serve as a guide 

 to the selection of the sounding interval: 



