782 HYDROGRAPHIC MANUAL PaGE 734 



Unless stated otherwise under the specific paragraphs, all explanatory notes placed 

 in the land area or in clear spaces in the water area shall be inked in black, but those 

 that must be placed in the sounded area should be left in pencil for subsequent inking 

 in the Washington Office, after verification of the survey. 



Periods shall not be used under any circumstances in connection with names, 

 notes, or any symbol that appears in the water area, as they might be confused with 

 bare rocks. (See also 783.) 



In addition to the types of notes referred to above and those specifically mentioned 

 elsewhere in this Manual, the following general classes of notes are required. These 

 apply mostly to control stations and landmarks, and should be shown in vertical 

 lower-case letters, and in parentheses (see fig. 171). 



(a) The name of each marked recoverable topographic station shall be followed by the word 

 ^'marked". 



(b) The name of each recommended landmark shown on the hydrographic sheet shall be followed 

 by the word "landmark", together with the elevations above the ground and above mean high water, 

 if known; and if the name is not self-explanatory, a few words describing the object should be added 

 (see 7844). 



(c) Each object in the water area used as a signal shall be explained by a note stating whether 

 it is of temporary or permanent nature, and if permanent its character must be described for the infor- 

 mation of the cartographer in charting (see 7444). 



(d) Each control station that is a natural or artificial object and is permanent or semipermanent 

 shall be briefly described in one or two words immediately following the station name. 



(e) If the source of the position of a control station is other than the official records of the Bureau, 

 an explanator}^ note shall be added after the station name to state the source of the data. 



782. Rocks, Reefs, Ledges, and Rocky Areas 



The representation of these features is one of the most important phases in the 

 plotting of a smooth sheet. Great care is necessary in order that there shall be no 

 ambiguity regarding their true character and that the conventional symbols used for 

 such features conform to the adopted standards in part "O" of the Symbols and Abbrevi- 

 ations chart (see fig. 189, part IX). 



In applying these features to the smooth sheet, those that originate solely with the 

 hydrographic survey or that result from an adjustment between the hydrographic and 

 topographic data should be left in pencil by the field party (see 7827). All other 

 features (and primarily these include information transferred from the topographic 

 survey) should be inked in black unless otherwise noted in this Manual. Similar 

 treatment should be followed in the case of descriptive notes or elevations accom- 

 panying such features. (See 7825 and table 28.) 



7821. Planes of Reference or Sounding Datums 



In the hydrography of the Coast and Geodetic Survey two general planes of refer- 

 ence (sounding datums) and one special plane are in use. They are mean low water 

 (MLW) for the Atlantic and Gulf areas, mean lower low water (MLLW) for the Pacific 

 areas, and mean low water springs (MLWS) for the Pacific entrance to the Panama 

 Canal (see 8224). The plane of mean high w^ater (MHW) is the datum to which the 

 shoreline and all land elevations are referred (see 2311). 



All rocks, reefs, and ledges shown on the smooth sheet must be referred to these 

 planes, and to no others, and they must be symbolized in accordance with the rules 

 given in 7823. 



