Page 541 echo sounding 563 



accurate corrections can be deduced from the depth contours based on uncorrected 

 echo soundings. The problem can be dealt with either by moving the echo sounding 

 to a point vertically over the echo point and reducing it to give that depth, or by increas- 

 ing it to give the vertical depth below the vessel. In either case, methodically correcting 

 the soundings on a sm-vey sheet is a laborious task. 



In areas of irregular bottom, it is not possible to reconstruct from uncorrected echo 

 soundings a profile or series of depth cm'ves of sufficient accuracy for this purpose. 

 The actual point on the bottom from which the echo for a particular sounding is re- 

 flected cannot be easily determined, and regardless of the intensity of a survey in deep 

 water, detailed contom*s drawn will not always disclose whether the survey has been 

 intense enough to justify the use of depth curves for the correction of echo soundings. 

 In a steep submarine canyon the echo of a sonic instrument would never come from the 

 deepest portion of the canyon as long as the distance from the survey vessel to the 

 sloping sides is less than the vertical depth below the vessel; in such a case there is no 

 practical correction that can be made. If the echo soundings are corrected on the 

 basis of the slope of the side of the canyon, there is always a likelihood of showing either 

 a greater or lesser depth than actually exists in the deepest part of the canyon, and sel- 

 dom will the true depth be indicated. 



It is therefore obvious that something more than a mere detailed contour sheet 

 based on the echo soimdings is required to make a rational analysis of the corrections 

 to be applied for slope to deep-water soundings in areas of irregular bottom. By 

 making certain assumptions, one Em'opean investigator has proposed the construction 

 of profiles of the individual sounding lines, with equal vertical and horizontal scales, 

 and from these and the contour sheet the corrections are to be deduced. This is an 

 elaborate method and its use depends on the closeness of the spacing of the sounding 

 lines. The method may find application in thoroughly sm-veyed regions where, for 

 scientific purposes, the closest possible approximation to the true bottom configuration 

 may be desired, which could not be obtained from the depth contours alone. This 

 would require, particularly in an area of nregular bottom, a much more intensive survey 

 than is needed for navigation and for general hydrographic surveys. The method 

 would not be justifiable. 



A practical solution to the problem would be to study the uncorrected echo sound- 

 ings in a given area, and where the.study discloses that the echo soundings by themselves 

 are not adequate for representing the bottom configuration, to supplement them by a 

 number of vertical soundings. This should be done on steep slopes contiguous to 

 gentle slopes and in submarine valleys and canyons. 



The nautical charts resulting from hydrographic surveys are made for the use of 

 the mariner. While, from a purely theoretical viewpoint, a correction should be 

 applied to echo soundings for errors caused by slope, there are several practical reasons 

 why this is neither desirable nor necessary. 



The depth at any point is represented on a chart by a numeral and not by a point. 

 From the viewpoint of the practical navigator then, there is no advantage in correcting 

 an echo sounding to verticality if at the scale of the chart an actual depth of water 

 equal to the uncorrected echo sounding would be found at a small horizontal distance 

 away, especially if this distance on the chart is normally covered by the height and width 

 of the numerals representing the sounding. In the practical application of this principle 

 it will be found that the corrections for slope will be automatically eliminated from a 

 large percentage of soundings. This is more fully explained in Special Publication No. 

 165, Slope Corrections for Echo Soundings. 



