Page 729 the smooth sheet 7753 



more than two systems the additional systems should be plotted on one or more overlay 

 tracings. 



If, for example, there are three systems of sounding lines, A, B, and C, in an area, system A should be plotted and the soundings 

 penciled directly on the smooth sheet. The position dots should be inked but the position numbers and day letters should be left in 

 pencil at this stage of the plotting. It is often advantageous to identify the positions temporarily by placing the numbers and day 

 letters well outside of the congested area, connected to their respective positions by lightly penciled leader lines. The second system, 

 B, if neither A nor B is too closely spaced, should then be plotted directly on the smooth sheet in the same manner as A. In penciling 

 the sou^idings of system B it will be found that some of these occur where soundings from system A have already been penciled, and it 

 will be necessary to erase some of the A soundings to make space for shoaler soundings obtained on the B system. The resultant sound- 

 ings on the smooth sheet should be an adequate selection from the A and B systems, depending on the depths obtained. No preference 

 should be given to the soundings of one system over the other. After the soundings have been penciled, the position numbers and day 

 letters can be inked where they will not Interfere with the soundings, using short leaders where necessary. 



The hydrography of system C should now be plotted on the overlay tracing, with the least depth found by that system empha- 

 sized by an arrow pointing to it, on which is lettered the depth, position number, and day letter. 



Finally, the least depth obtained from all of the systems on any given shoal must be plotted on the smooth sheet and indicated 

 by a leader at whose end in a clear space is given the depth, and the position number and day letter (see 7753). 



All overlay tracings must be forwarded to the Washington Office with the smooth 

 sheet. Wlien the latter is verified in the Office the positions will be transferred from the 

 overlays to the smooth sheet, and a selection of soundings made which will adequately 

 represent the depths in the area; after review of the smooth sheet the overlays will be 

 destroyed. 



7753. Least Depths 



The least depth on each shoal, whether or not it contains intensive development, 

 should always be shown slightly larger and bolder than the surrounding depths, in 

 order that it will not be overlooked even in a cursory examination of the smooth sheet. 

 (See 7732.) And in all cases where there is any doubt about the least depth being 



readily noticeable, there should be added, in pencil, a legend "Least depth ft" 



in a clear space (on the land area if necessary) with a fine arrow, or leader, drawn toward 

 the minimum depth. The position number and day letter should also be given if not 

 otherwise clear. 



If available, a bottom characteristic should always be added as near the least depth 

 as practicable. 



The same general procedure shall be followed when such least depths are inked in 

 the Office, except that the position numbers and day letters will ordinarily not be inked 

 in the legend. 



776. Depth Curves 



Depth curves, or curves of equal depth, are shown on the smooth sheet for the pur- 

 pose of bringing clearly to the eye the general configuration of the bottom, and for 

 emphasizing important navigational features, such as shoals and channels. They are 

 also of value in studying the adequacy of the survey in the Office, where areas are often 

 discovered that require additional field examination. 



Depth curves are comparable to contours on land, each curve representing an 

 imaginary line on the ground (in the water area), every point of which is at the same 

 depth below the sounding datum. The principles which govern the delineation of land 

 contours are equally applicable to the drawing of depth curves, and a knowledge of 

 topographic expression and submarine relief is essential for their correct representation. 

 A study of the characteristic bottom forms in any region is of value in the interpretation 

 of hydrography, as such forms usually repeat themselves in similar regions, and often 

 in the same region. (See 3531 and 355.) 



Abnormal or improbable depth curves are strong evidence of probable uncertainties 

 or inaccuracies in the hydrographic survey or the reduction of records, and the soundings 



465382—44 48 



