38 



HYDROGRAPHIC MANUAL 



destroyed or moved shall be selected. The 

 use of day beacons, range marks, targets, 

 etc., should be avoided. 



The objects selected shall be plotted on a 

 copy of the latest print of the largest-scale 

 chart of the area. A short legend and the 

 date of the field survey or number of the 

 topographic sheet of the area shall be shown 

 on the chart. The three or four objects se- 

 lected for each fix shall be encircled in red 

 on the chart, whether already charted as 

 landmarks or plotted by the survey party. 

 Adjacent to the position of each object se- 

 lected, a description from which the object 

 may be identified shall be noted in red. The 

 fix, and check angle, selected for each aid 

 shall be indicated on the chart by red arrows 

 at the aid, pointing toward the object to be 

 used. 



The prepared chart, after the information 

 thereon has been carefully verified, shall be 

 forwarded directly to the Commander of the 

 Coast Guard District in which the area is 

 located, with a transmitting letter stating 

 its purpose. Explanatory information for 

 proper interpretation of the data on the chart 

 shall be included if necessary. 



The following data should be sent to the 

 Washington Office: 



(a) A copy of the transmitting letter. 



(b) A list of the objects plotted on the 

 chart with their latitude and longitude and 

 method of location. The list shall be pre- 

 pared on Form 567, Landmarks for Charts, 

 and should include only those objects not al- 

 ready charted. The title on the form should 

 be cut off, or marked out, and a new heading 

 "Objects for use of U.S. Coast Guard" writ- 

 ten in. 



The transmittal of these data shall be han- 

 dled as a separate subject, and no other field 

 information or notes shall be included in the 

 envelope. 



2-49 Tide stations. — Tidal data are re- 

 quired for determination of the various 

 datum planes (see 1-45), for prediction of 

 tides, and for use in a hydrographic survey. 

 Tide stations shall be established in the im- 

 mediate vicinity of the hydrographic opera- 

 tions in order that the soundings may be 



accurately reduced to the sounding datum 

 (see 5-102) . The number of stations required 

 depends on the character of the area ; where 

 the flow of tide is restricted, numerous 

 gages may be required to determine accu- 

 rate tidal constants (see 1-46) . The selection 

 of sites for the tide gages to be used in 

 hydrographic survey is usually made in the 

 Washington Office and is specified in the 

 project instructions. This prior selection does 

 not relieve the Chief of Party from a re- 

 sponsibility to make a study of the tidal 

 regime of the project area. A discussion of 

 locations of secondary tide stations is con- 

 tained in paragraphs 168 to 177 in Special 

 Publication No. 196, Manual of Tide Observa- 

 tions. When unusual conditions of land con- 

 figuration or weather greatly affect the tide, 

 extreme care is required to avoid large er- 

 rors in tide reducers. 



2-50 General plan of tide stations. — In 



most cases, an existing primary tide station 

 in the general vicinity of the project will 

 be used as the control station. In other 

 cases, a standard automatic gage shall be in- 

 stalled at a central point to serve as a control 

 station and shall be maintained in operation 

 during the entire period covered by the sur- 

 vey. As the work progresses, secondary tide 

 stations are established at other places in 

 the immediate locality being sounded. When 

 practicable, observations at each secondary 

 station shall be continued over a period of 

 not less than 29 days. Adjacent secondary 

 tide stations in a project area should be op- 

 erated simultaneously for a period of ap- 

 proximately 2 weeks, if practicable, espe- 

 cially when time or range factors change 

 appreciably. A portable automatic tide gage 

 is usually used for this purpose. Detailed 

 descriptions of the tide gages with full in- 

 structions for their installation and opera- 

 tion are contained in Special Publication 

 No. 196. 



2-51 Bench marks and leveling. — At each 

 tide station there must be a tide staff con- 

 nected by levels with at least three substan- 

 tial bench marks, so located that they will 

 not all be likely to be destroyed by a common 



