Page 739 the smooth sheet 7831 



accompany. Where such placement interferes with the chirity and legibility of the 

 sounding, the bottom characteristic should be placed in any convenient space in the 

 immediate vicinity of the accompanying sounding. The abbreviations should be 

 lettered in single stroke slanting letters, the capital letters having an average height 

 slightly less than the soundings and the lower-case letters correspondingly smaller. 

 (See fig. 163.) 



After the smooth sheet has been verified, the bottom characteristics will be inked 

 in black in the Washington Office. At that time, if it appears that an insufficient 

 number were obtained, the new survey may be supplemented, on direction of the Chief 

 of Surveys Branch, with bottom characteristics from prior surveys (see 3842 and 

 93336). 



7831. Silted Areas 



The fathograms of echo-sounding instruments operating at supersonic frequencies 

 show quite clearly, under certain conditions, layers of silt or other loosely distributed 

 sediment overlying the substrata. These are commonly found in bays, lakes, or 

 estuaries, where silt-laden streams have deposited their loads, or in offshore basin areas 

 where unusual conditions may permit the deposition of silt. 



A record of the existence and limits of such sedimentary layers is often of con- 

 siderable importance. Where encountered in an area being surveyed, the Washington 

 Office shall be notified of the apparent features and special instructions will be issued 

 if a smooth-sheet record is desired. 



In such cases the silted areas shall be noted on the smooth sheet, at the time the 

 soundings are plotted, and the approximate outline of each area more than 1 inch 

 in diameter or 1 inch wide, shall be shown by a dash pencil line. A notation explain- 

 ing their significance shall be made on the title sheet and in the Descriptive Report. 

 The outlines will be inked in a brown dash line after verification in the Washington 

 Office, and at the time of inking the depth curves. 



Where silted areas are so frequent that they will be confusing on the smooth sheet 

 or may be confused with the depth curves, their limits shall be shown in brown dash 

 ink lines on a tracing-cloth overlay. The overlay shall be referenced to the smooth 

 sheet by projection intersections throughout the area and shall be identified by a title, 

 as "Overlay showing silted areas on H-5236." A notation shall be placed on the title 

 sheet (Form 537) that such an overlay accompanies the smooth sheet. 



Where silted areas are disclosed by the fathograms a discussion of their estimated 

 frequency, sizes, and apparent thicknesses shall be made in the Descriptive Report, 

 irrespective of whether their outlines are shown graphically on the smooth sheet or on 

 an overlay. 



784. Aids to Navigation 



All aids to navigation, fixed and floating, within the area of the survey, shall be 

 shown on the smooth sheet. The positions of fixed aids located by triangulation shall 

 be shown by standard triangulation station symbols; where located by topographic or 

 hydrographic methods, control station symbols shall be used if the aids were so used 

 during the hydrographic survey; otherwise, nautical chart symbols should be used 

 (see fig. 169). The positions of floating aids, irrespective of method of location, shall 

 be indicated by the actual aid symbols and colors used on the nautical charts of the 

 Bureau. (See part ''L" of Symbols and Abbreviations chart, fig. 189, part VIII.) 



