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HYDROGRAPHIC MANUAL 



they should be reported immediately by ra-. 

 die to the Commander of the nearest U.S. 

 Coast Guard District. 



5-73 Discrepancies at crossings. — Cross- 

 lines are intended to disclose discrepancies 

 in soundings resulting from various causes 

 (see 1-26). The discrepancies may be sys- 

 tematic or accidental. They must be recog- 

 nized as evidence of some fault of the ap- 

 paratus, method, or record, requiring a study 

 to discover the source and to indicate the 

 most probable correction. 



The allowable differences in depths at 

 crossings in any area should be based on 

 the amount of horizontal displacement cor- 

 responding to the differences in depth, rather 

 than a percentage of the depth. In com- 

 paratively even bottom a difference of 2 or 3 

 feet may be excessive because of the lateral 

 displacement of the depth curve. In areas 

 of irregular bottom or on steep slopes dif- 

 ferences of several feet or fathoms may be 

 allowable since the depth curves will not be 

 appreciably affected. Allowable differences 

 at crossings on the smooth sheet are specified 

 in 6-61 (d). Since minor corrections are 

 ignored and predicted tides are used when 

 inking soundings on the boat sheet, greater 

 differences may be expected. In areas of 

 smooth bottom and depths less than 11 fath- 

 oms, the discrepancies should not exceed 2 

 feet or 0.4 fathom. In areas of irregular 

 bottom and in depths greater than 11 fath- 

 oms, discrepancies should not exceed 3 per- 

 cent in the lesser depths and should decrease 

 to 1 percent or less in ocean depths. 



When the discrepancy is consistent at a 

 number of successive crossings, and the con- 

 trol is strong, it is probable that the echo 

 sounder is at fault or that the plane of ref- 

 erence is incorrect. When Shoran control is 

 used and a moderate displacement of the 

 sounding line would bring the hydrography 

 into agreement, the calibration of the Shoran 

 should be suspect and should be repeated. 



Several vertical cast comparisons must be 

 made in the vicinity of the crossings to sup- 

 port the conclusions reached by a study of 

 the recorded data (see 5-75). 



5-74 Discrepancies at junctions and 



overlaps. — When inshore launch hydrog- 

 raphy is overlapped by the ship at the off- 

 shore limits of the launch work, soundings 

 at the junction sometimes fail of agreement. 

 Since it is not practicable to obtain bar 

 checks on a ship, reliance must be placed on 

 vertical cast comparisons to determine the 

 instrumental error of the echo sounders used 

 aboard ship. Several comparisons shall be 

 made in the overlap area to provide data for 

 reconciliation of possible discrepancies. 



A similar situation exists when the hy- 

 drography accomplished by different launches 

 is joined on the inshore sheets. If there is a 

 displacement of the depth curves at the junc- 

 tion it is probable that an error exists in 

 the work of one or both launches and the 

 amount and source of the error must be 

 established by comparison of both echo 

 sounders with vertical cast soundings. 



5-75 Other discrepancies in hydrography. 



— On occasion other discrepancies are found 

 which are less obvious and more difficult to 

 explain or correct. After the regular system 

 of lines has been run over a wide area, it is 

 sometimes necessary to reduce the spacing 

 at the inshore ends of the lines in order to 

 comply with instructions. The soundings on 

 the alternate lines may differ by two to three 

 feet in areas of comparatively flat bottom 

 and the soundings on one system of lines 

 are obviously in error. In other instances 

 the same launch has used different echo 

 sounders on alternate days and the sound- 

 ings obtained with each instrument are 

 consistent within themselves but fail of 

 agreement with those from the other. 



In such cases, the hydrographer must 

 make a field test to determine which sound- 

 ings are in error and, if possible, determine 

 the cause of the discrepancy and eliminate 

 it. The source of these types of discrepan- 

 cies is usually found in the echo sounder or 

 the method of operation. On 808 type echo 

 sounders the operator may have the initial 

 cutout switch in the on position for opera- 

 tion in shoal water and the initial setting 

 may be incorrect; or the motor speed may 

 not be correct. The vibrating reed is subject 

 to movement by extraneous vibrations and 



