7741 



HYDROGRAPHIC MANUAIi 



Page 726 



(c) An inaccurately calibrated sounding apparatus or the misapplication of the corrections, among 

 which may be the following: 



(1) Leadline not compared with the standard. 



(2) Leadline varied in length between standardizations. 



(3) Sounding machine registering sheave 

 not correctly calibrated. 



(4) An erroneous velocity of sound used 

 in the reduction of echo soundings. 



(d) Erroneous application of any of the data 

 used in reducing the soundings. 



(e) Large clock errors which may result in an 

 erroneous tide reduction. 



(/) An inaccurate plane of reference; errors 

 from this source may occur where — 



(1) The tide gage is located at too great 

 a distance from the area being sounded. 



(2) The area being sounded is blocked by 

 shoals from free access of the tide. 



(3) The tide gage is so located that the free 

 rise and fall of the tide, particularly the low 

 water, is not correctly recorded. 

 ig) A rough state of the sea — -errors due to 



this condition may be detected where one day's 

 work performed in rough weather is adjacent to 

 another day's work performed in calm weather; or 

 where crosslines have been run in an area imder 

 different conditions of sea from those which ob- 

 tained at the time the principal system of lines 

 was run. 



(h) Where the leadline or wire is not perfectly 

 vertical — such condition occurs when the sound- 

 ing line is run in a wind, sea, or current, handlead 

 soundings being particularly vulnerable in this 

 respect (see 3464) ; wire soundings in deep water 

 may be similarly affected because of the impossi- 

 bility or impracticability of maintaining the posi- 

 tion of the vessel vertically over the lead (see 3422). 



(i) Where handlead or wire soundings are 

 taken in an area of very soft or oozy bottom, where 

 it is difficult or quite impossible to detect when the 

 lead strikes the top of the soft layer. 



(j) Where soundings are taken on abrupt slopes ; 



(1) In handlead or wire soundings the 

 lead may slip down a rocky slope. 



(2) In echo sounding, the sound may be 

 reflected from the side of the slope rather than 

 from vertically beneath the vessel (see 563) . 



(3) A small displacement of the position 

 Figure 164.-Right-hand page of Sounding Record with of the sounding may affect the depth by a 



corrections properly made (reduced about one-half). considerable amount. 



7741. Corrections to Recorded Data 



The instructions for recording the data forbid erasures, and recorders correct their 

 •errors by crossing out the erroneous entry and writing the corrected one above or at one 

 side (see 81). The smooth plotter may frequently find that the rejected entry, or some 

 part of it, was correct, but that its application may not have been correct. 



