16 



HYDROGRAPHIC MANUAL 



1-42 Bottom characteristics. — The char- 

 acter of the bottom shall be determined by 

 sampling with corers, snapper cups, scoop- 

 fish, or armed leads at regular intervals. On 

 inshore surveys the distance between sam- 

 ples should not exceed 21/^ inches at the 

 scale of the survey, and in depths less than 

 100 fathoms in offshore areas the distance 

 should not exceed 5 inches. Between the 100 

 and 1,000-fathom curve, the character of the 

 bottom shall be determined at intervals of 

 about 5 to 10 miles. In greater depths bot- 

 tom samples shall be obtained at all oceano- 

 graphic stations and at such other places 

 as the project instructions specify. In har- 

 bors and anchorages the information should 

 be sufficiently complete to define the approxi- 

 mate limits of each type of bottom (see 5-76) . 



If the project area has been surveyed pre- 

 viously and bottom characteristics were ade- 

 quately determined, the above spacing may 

 be doubled unless changes in the bottom 

 characteristics or depths are discovered. The 

 abbreviations shown in 5-88 shall be used to 

 record and plot bottom characteristics. 



1-43 Inspection of boat sheet and rec- 

 ords. — The Chief of Party should inspect the 

 boat sheets and records at regular intervals, 

 daily if possible, to assure himself that all 

 operations are in accordance with the re- 

 quirements contained in applicable manuals 

 and the project instructions. When making 

 his examinations, particular attention must 

 be given to the adequacy and completeness 

 of the survey with special reference to ex- 

 amination of indications of shoals, determi- 

 nation of least depth on submerged rocks, 

 shoals, bars, and wrecks, and the develop- 

 ment of navigable channels. He should de- 

 termine that junctions with adjacent surveys 

 are satisfactory and that no unsurveyed 

 gaps are left in the area (see 5-66). After 

 this review of the records, the Chief of 

 Party should indicate to the hydrographer 

 where additional work is required, and if 

 necessary, correct any unsatisfactory meth- 

 ods or procedures being used. 



After the survey is completed and prior 

 to departure from the project area, the Chief 



of Party should make a final inspection of 

 the sheet. All questions of adequacy or com- 

 pleteness of the survey should be resolved 

 before leaving the area. The boat sheet 

 should also be examined for clarity since it 

 will be used in the Washington Office for 

 preliminary revision of the chart. 



1-44 Forwarding boat sheets. — Boat 

 sheets for all surveys shall be forwarded to 

 the Washington Office for copying as soon 

 as practicable after the survey is completed 

 and not later than one month after the close 

 of a field season. The sheet should be accom- 

 panied by a tabulation of important changes 

 found by a preliminary comparison with the 

 largest scale chart of the area. 



Where boat-smooth sheet methods are used 

 to plot offshore hydrography, the cover sheet 

 need not be forwarded in advance if the 

 smooth sheet is to be completed within 3 

 to 4 months. 



1-45 Planes of reference. — The planes of 

 reference adopted for the reduction of sound- 

 ings and the publication of charts of the 

 Coast and Geodetic Survey are as follows: 



(a) For the Atlantic Ocean and Gulf 

 of Mexico — the mean of the low waters 

 (MLW) ; 



(b) For the Pacific Ocean — the mean of 

 the lower low waters (MLLW) ; 



(c) For certain of the larger navigable 

 rivers and lakes special planes have been 

 adopted. In such cases the project instruc- 

 tions will specify the plane of reference. 



1-46 Tide stations. — If there is no pri- 

 mary tide station in or near the project area 

 which will serve as a control station, a stand- 

 ard automatic gage shall be installed at a 

 central point and operated during the entire 

 period covered by the survey. As the work 

 progresses, secondary tide stations shall be 

 established at other places in the immediate 

 locality being sounded. The selection of sites 

 for the tide gages to be used in a hydro- 

 graphic survey is usually made in the Wash- 

 ington Office and is specified in the project 

 instructions. The number and distribution 

 of stations will depend on the character of 



