5. HYDROGRAPHY 



175 



Form 116 for reducers entered in fathoms. 



In some areas it is necessary to use tide 

 zones between adjacent tide stations be- 

 cause of time or range differences. Proce- 

 dures for making time and height allow- 

 ances are explained in Special Publication No. 

 196, Sections 255-258. Tide zones are estab- 

 lished by the Washington Office on request. 

 Zoning is often critical in estuaries and in 

 long narrow bays having a large range of 

 tide. Improper zoning in such areas is re- 

 flected in the junctions with adjacent sur- 

 veys and may cause excessive differences in 

 depths at crossings within the survey (see 

 l-26e). 



5-103 Leadline corrections. — The lead- 

 line shall be compared with a standard 

 length each day it is used and the results 

 recorded in the sounding record under the 

 headings in rubber Stamp No. 35. All lead- 

 line soundings for that day shall be cor- 

 rected, if necessary, in accordance with the 

 comparison (see 3-67). The corrections are 

 entered in the column headed "Leadline." 



5-104 Wire sounding corrections. — The 



registering sheave used in wire sounding 

 shall be calibrated in accordance with 3-73 

 and all wire soundings shall be corrected 

 if necessary. To correct wire soundings, mul- 

 tiply the observed depth by the sheave factor, 

 apply the tide correction, and enter the result 

 in the column headed "Field." 



5-105 Echo-sounding corrections. — Echo- 

 sounding instruments are intended to be ad- 

 justed and operated so that the velocity and 

 tide corrections are the only corrections that 

 need to be applied to echo soundings. It is 

 sometimes necessary to apply other correc- 

 tions such as variation of the initial from 

 the adopted setting, incorrect speed of op- 

 eration, and similar instrumental errors or 

 faults of operation. Each source of error is 

 discussed in the following sections. 



When sounding operations are in prog- 

 ress, the hydrographer may become so en- 

 grossed with measurements in a horizontal 

 plane that the problems involved with meas- 

 urements of the depth are neglected. The 

 fact that an echo sounder appears to be op- 



erating correctly is not always sufficient evi- 

 dence that the recorded soundings are cor- 

 rect. The detection of errors in positioning 

 and sounding, and the accumulation of data 

 for the correction of these errors presents a 

 real challenge in many instances. The final 

 measure of the quality of the work is found 

 here. 



Occasionally inconsistencies are not dis- 

 covered until the survey is smooth plotted. 

 The available data should be adequate to 

 permit a reasonable solution of the prob- 

 lems and a proper adjustment of discrepan- 

 cies. The following examples indicate the 

 variety of problems encountered in smooth 

 plotting and verifying a survey. 



A launch was equipped with two echo 

 sounders, one of which was used to obtain 

 soundings on the regular system of lines. 

 The second instrument was used at a later 

 date to run cross lines and closely spaced 

 lines for development of shoal indications. 

 The bar checks taken with each instrument 

 were satisfactory, but recorded soundings 

 were consistently different by 2 feet. There 

 were no vertical cast comparisons with ei- 

 ther instrument. There is no evidence to 

 prove which soundings are correct. 



At the junction of hydrography accom- 

 plished by a launch with that done by an- 

 other vessel, the differences in depths range 

 from 2 to 5 feet with a fairly consistent dif- 

 ference of 3 feet. The launch made bar check 

 observations at a remote area but not in the 

 area covered by the hydrography. Neither 

 vessel recorded a vertical cast comparison. 

 Obviously, the soundings recorded by at 

 least one unit are in error, perhaps both are 

 incorrect. 



As a general rule, the accuracy of sound- 

 ings obtained by portable echo sounders is 

 directly related to the competence and re- 

 liability of the technicians who operate the 

 instruments. Each type of echo sounder has 

 characteristic features which must be closely 

 watched, such as voltage, gain, paper speed, 

 phase, paper alignment, initial setting, fre- 

 quency, and stylus length. Failure to main- 

 tain proper adjustments results in erratic 



