7723 HYDROGRAPHIC MANUAL Page 720 



be made for each R.A.R. sheet and included in the Descriptive Report (see 8i2K). 

 This will give some idea of the relative accuracy with which the hydrography has been 

 controlled, if the depths are accurate. This tabulation can be most conveniently made 

 as the soundings are plotted on the sheet. At each crossing the depth differences are 

 computed and when these data are available for the entire sheet, percentages are com- 

 puted which will give a reasonably fair idea of the accuracy with which the survey has 



been controlled. 



7723. Spacing Echo Soundings 



Because of the possibility of obtaining an almost infinite number of soundings when 

 the echo method is used, there are frequently more soundings recorded than can or 

 should be shown on the smooth sheet. Echo soundings are spaced in the conventional 

 manner, except that a certain percentage of those recorded will often have to be omitted 

 from the smooth sheet. In echo sounding, soundings are frequently recorded at irregu- 

 lar intervals, but these are usually more important than those recorded at regular 

 intervals, because they define irregularities in the general slope of the bottom. (See 

 3423.) 



Where soundings are penciled on the smooth sheet directly from a fathogram, clock 

 time is not necessarily used to determine the spacing. The printed fathograms used on 

 the 808 Fathometer contain equally spaced vertical arcs, by which the spacing of the 

 soundings may be determined. When a fathogram is used, the three-point fixes are 

 often taken at the exact moment the stjdus is marking on one of the vertical arcs, and 

 irrespective of clock time, with the interval between positions being determined by the 

 number of spaces between position arcs rather than by the number of minutes and 

 seconds. When soundings from such a record are to be transferred to the smooth sheet, 

 the spacing dividers should be adjusted between any two consecutive positions on the 

 smooth sheet so that each divider point represents a vertical printed arc on the fatho- 

 gram, if the scale permits. Wlien this is done each divider point on the smooth sheet 

 represents the position of the vessel when the sounding was recorded on one of the 

 vertical arcs. In any case, the divider points should be adjusted to a definite relation 

 with the printed arcs, rather than to the clock time, as is necessary for soundings 

 recorded in a Sounding Record. Wliere tliree-point fixes have been obtained which 

 did not coincide with one of the vertical arcs, the method is similar except that fractional 

 parts of the spaces between the position arcs must be considered in making the spacing 

 divider adjustment on the smooth sheet in relation to the positions, so that each 

 divider point will still represent a printed arc. 



Mechanical spacing of soundings derived from a fathogram is not to be tolerated. 

 Selection should be made as described in 7726, plotting the soundings on the smooth 

 sheet in their correct positions between the points of the spacing dividers by reference 

 to their positions between corresponding vertical arcs on the fathogram. 



7724. Frequency of Soundings 



The scale of the smooth sheet and the sounding interval will ordinarily be selected 

 so as to permit all soundings recorded in the Sounding Record to be shown on the smooth 

 sheet. The frequency of recorded soundings is, of course, greater in shallow than in 

 deep water, and this may result in more soundings being recorded than can be shown 

 on the smooth sheet. In this case selection is necessary (see 7725). 



