Page 203 hydrography 3153 



In important harbors, anchorages, channels, and areas of irregular submarine 

 relief, lines should generally be spaced 50 meters apart, and even closer spacing may- 

 be required for complete development in certain areas. 



3153. Lines Parallel to the Shore 



In general, hydrographic surveys should be extended inshore across the low- 

 water line in areas where this is practicable and can be done without danger to the 

 personnel or equipment (see 3122 and 3532). These inshore lines shoidd generally 

 be run parallel to the shoreline. The spacing of a system of lines paralleling the shore 

 shall increase gradually from a minimum next to the shore to a maximum between 

 the two outermost lines. The two inshore lines shall be not more than 50 meters 

 apart and the spacing between the two outermost lines should be that required for the 

 depths found. Depending on the depths and the number of lines to be run parallel 

 to the shore, a spacing increased 50 to 100 percent for each line should give an adequate 

 development. For example, a 50-percent increase would give successive lines spaced 

 50, 75, 110, 165, 250, and 375 meters apart. 



3154. Shoal Indications 



The spacing adopted for the regular system of sounding lines should be such 

 that the depths obtained will almost always give an indication, at least, of every 

 existing danger or shoal. It should not be assumed that the least depths will be 

 obtained on the regular system of lines. Every sounding of a depth slightly less than 

 the surrounding average depth should be regarded as a definite indication of a possible 

 shoal; this evidence is greatly increased where shoaler soundings occur on adjacent 

 lines in the same locality. These indications must be adequately developed by more 

 intensive measures. (See 355 and 36.) 



32. THE BOAT SHEET 

 321. Definition and Purpose 



The boat sheet is the work sheet used by the hydrographer in the field for plotting 

 the details of a hydrographic survey as it progresses. Its use enables the hydrographer 

 to cover an area with lines of soundings in a systematic and economic manner, to 

 judge the adequacy of the survey, and to ascertain where additional development or 

 investigation is required. The boat sheet is similar to the smooth sheet (see chapter 7) 

 but necessarily less accurate because of the haste with which the hydrographer must 

 plot each position during survey operations, because of the lack of adequate tidal data 

 for reducing the soundings, and because of the exposure to the weather to which the 

 sheet is subjected. 



On the boat sheet the hydrographer plots the successive positions of the sounding 

 vessel as they are observed and is thus able to determine whether the area is being 

 systematically and adequately covered. The soundings corrected to approximate 

 sounding datum by the use of predicted tides are also inked on the boat sheet daily 

 during the progress of the field work. They disclose to the hydrographer uneven 

 bottom which may require additional development to determine the least depths on rocks 

 or shoals, and they indicate areas where additional sounding lines must be run so that 

 the depth curves can be drawn with certainty. The boat sheet serves also as a guide 

 to the cartographer during the plotting and verification of the smooth sheet and aids 

 in clarifying uncertainties which may arise at such times. 



