323 HYDROGRAPHIC MANUAL PaGE 206 



If the limits of the hydrographic survey extend beyond those of the topographic survey, 

 the transferred portion of the projection may be extended to the limits of the boat sheet. 



In the rare case when a smooth sheet is prepared first, a boat sheet may be con- 

 structed by superimposing the smooth sheet on the boat sheet and pricking the projec- 

 tion intersections and control points through with a fine needle, other details being 

 transferred by tracing. This same method may be used with a topographic survey 

 which is not on an aluminum-mounted sheet (see 233) . 



The accuracy of construction obtained b}^ the use of the projection ruling machine 

 (7327) is not warranted in a boat sheet, and requests should not be made to the Wash- 

 ington Office for their construction. 



When the area to be sounded has been preceded by an air photograpliic survey, 

 boat sheets will sometimes be printed or prepared in the Washington Office by one of 

 the methods described in 733. 



323. Preparation of Boat Sheet 



A hydrographic survey can be made with a boat sheet on which appear only the 

 positions and names of the control stations, but for an efficient and complete survey, 

 the details described in 3231 to 3237 should be placed on the boat sheet for each area 

 before the survey of that part is started. 



3231. Control Stations 



All control stations, whose positions are knoAvn at the start of the survey, should 

 be plotted on or transferred to the boat sheet. 



Names of control stations may be lettered on the boat sheet in freehand, provided 

 they are unmistakably legible. Existing names of control stations, such as triangu- 

 lation stations, marked topographic stations, and previously named topographic 

 or hydrographic stations, must be retained with their exact spelling and should be 

 lettered when the control is plotted. New station names are to be assigned by the 

 hydrographer in accordance with 215. They may be assigned to all the unnamed 

 stations and lettered on the boat sheet when the control is plotted, or during the 

 progress of the survey as each station is used. 



Where the control stations are numerous, as on an inshore hydrographic sheet, 

 identification will be aided and confusion avoided if descriptions of those having a 

 characteristic appearance, either in form or color, are written on the boat sheet. 



For each control station in the water area, notation should be made on the boat 

 sheet as to whether the feature on which it is erected is permanent or temporary, and 

 a short description of the feature should be added ; for example, that it is a pile, a rock, 

 a shoal spot, a menace to navigation, or a signal erected temporarily by the hydro- 

 graphic party. 



Each control station that is a natural object shall likewise be briefly described. 



If conspicuous enough for use as a landmark, that fact shall be included in the 



description. 



3232. Topographic Details 



A boat sheet for an inshore hydrographic survey should contain the high-water 

 line, the low-water line, the approximate limits of shoal areas, rocks (bare, awash, and 

 sunken), aids to navigation, and any suspected dangers which the topographer may 

 have noted but may not have been able to verify or locate. The scale of an inshore 

 hydrographic survey is almost invariably the same as that of the corresponding topo- 



