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HYDROGRAPHIC MANUAL 



may occur. If errors are discovered or sus-s 

 pected, they must be verified and corrected 

 whenever necessary. No deviation from 

 original recorded data shall be made unless 

 this seems reasonable and is supported by 

 other evidence. Some of the more common 

 causes of discrepancies found in smooth plot- 

 ting are : 



(a) Fathograms may be improperly or in- 

 adequately scanned. This may be the result 

 of even-interval scanning and omission of 

 peaks and deeps; or improper interpretation 

 of echoes from marine growth, strays, and 

 side echoes. 



(b) An unnatural depth may result from 

 a; 5- or 10-fathom error in scanning or from 

 an error in reduction of the sounding. On 

 overlaps and crosslines it may result from 

 even-interval recording of a sounding which 

 should have been recorded at a fraction of 

 an interval, or from incorrect spacing of 

 soundings across the side of a channel or 

 other steep slope. An anomalous depth may 

 result from an unrecorded departure from 

 a straight course between positions near the 

 edge of a channel or bank, but the offset 

 sounding should be disproved by overlap- 

 ping hydrography before it is rejected. De- 

 tached leadline or wire soundings obtained 

 incidental to sampling the bottom may be 

 erroneous for various reasons. 



(c) There are many reasons why a line 

 of soundings might disagree with other hy- 

 drography. If the differences in depths vary 

 inconsistently, particularly if the course ap- 

 pears erratic, the discrepancy usually origi- 

 nates with the control. If the differences are 

 generally consistent then they may be due 

 to one factor or a combination of several, as 

 unrecorded wind tides ; echo penetration or 

 shoaler reflection of soft bottom caused by 

 variations in gain or by uncorrected varia- 

 tions in two echo sounders; echo sounding 

 versus leadline or pole sounding in soft bot- 

 tom ; changes in the initial on the fathogram ; 

 errors in phase correction ; deficiencies in 

 the calibration of echo sounders ; omissions of 

 or deficiencies in settlement and squat cor- 

 rections of the survey ship or launch ; or 

 erroneous scanning of saw-tooth profiles. 



(d) Where considerable hydrography is 

 in disagreement, the discrepancies may be 

 caused by one or more of the following: in- 

 accuracy in sounding line control which may 

 be caused by inadequate calibration of elec- 

 tronic control or incorrect application of 

 calibration corrections; errors in the posi- 

 tions of signals or weak fixes ; inaccuracies 

 in the soundings obtained by one or both 

 sounding vessels in the junction area or 

 by more than one vessel working on the same 

 survey ; inadequate tidal information or in- 

 correct zoning between tide stations. 



Since data may be transferred to the chart 

 from the smooth sheet before it has been 

 verified and reviewed, all discrepancies shall 

 be resolved insofar as practicable before the 

 sheet and records are forwarded to the office. 

 In areas of flat or gently sloping bottom and 

 depths less than 11 fathoms, discrepancies 

 of one unit in feet or 0.2 unit in fathoms 

 can be expected occasionally, and, except 

 where these differences affect a natural de- 

 lineation of depth curves, they do not jus- 

 tify extensive investigation. The report on 

 smooth plotting shall include references to 

 all significant unresolved discrepancies. 



6-62 Bottom characteristics. — All record- 

 ed bottom characteristics shall be penciled on 

 the smooth sheet except when an excessive 

 number of them are recorded on pole or 

 leadline surveys. In the latter case a selection 

 should be made being careful to plot all iso- 

 lated rocky or hard bottom characteristics. 

 On an ideal survey of important waters, such 

 as harbors and anchorages, the plotted 

 characteristics should be adequate to define 

 the approximate limits of various types of 

 bottom in the area. On some echo-sounding 

 surveys where bottom characteristics have 

 not been determined with sufficient frequency 

 (see 1^2), and where prior surveys have 

 been made, characteristics will be trans- 

 ferred in color from the prior survey during 

 office review in areas where no changes in 

 depth have occurred. 



Standard abbreviations for bottom char- 

 acteristics have been adopted (see 5-88) and 

 shall be used on the smooth sheet. The first 

 letter of the nouns shall be capitalized 



