1431 HYDROGRAPHIC MANUAL PaGE 32 



datum. 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 accurate 

 tidal constants. 



1431. Sites for Gages 



The selection of sites for the tide gages to be used in a hydrographic survey is usu- 

 ally made in the Washington Office and is specified in the project instructions. If on 

 arrival at the working ground the selected sites are found to be impracticable, the Chief 

 of Party may make necessary substitutions, but he must advise the Office of such 

 changes and the reasons for making them. 



1432. General Plan for Tide Stations 



In establishing tide stations in an area, the general plan is to install a standard 

 automatic gage at a central point to serve as a control station to be maintained in 

 operation during the entire period covered by the survey. In some cases, an existing 

 primary tide station in the general vicinity of the project may be used as the control 

 station. As the work progresses, secondary tide stations are established at other places 

 in the immediate locality being sounded, the distribution depending on the change in 

 the tide from place to place. When practicable, observations at each secondary station 

 shall be continued over a period of not less than 2 weeks. A portable automatic gage 

 is usually used for this purpose. A discussion of locations for the secondary tide stations 

 is contained in paragraphs 168 to 177 in Special Publication No. 196, Manual of Tide 

 Observations. Detailed descriptions of the tide gages with full instructions for their 

 installation may be found in the same publication. Officers must thoroughly familiarize 

 themselves with both types of gages and their operation. 



1433. Bench Marks and Lereling 



At each tide station there must be a tide staft' connected by levels with at least 

 three substantial bench marks, so located that they will not all be likely to be destroyed 

 by a common cause. Search shall be made for all old bench marks in the general 

 vicinity, their present condition shall be noted, and the old descriptions revised if 

 necessary. So far as practicable there should be maintained in every locality where a 

 tide staff is installed, or where tide observations have previously been taken, at least 

 five bench marks in good condition. Any deficiency found at the time of visiting such 

 a site should be corrected by the installation of new bench marks. New bench marks 

 should be clearly described so that they may be readily recovered and identified. All 

 the bench marks recovered and established should be connected by levels with the 

 tide staff. 



In assigning numbers to new bench marks, the duplication of numbers previously 

 used in the same locality for other bench marks, whether destroyed or extant, shall be 

 avoided. When a bench mark of another organization is recovered and connected by 

 leveling with the Coast and Geodetic Survey bench marks, the number or name assigned 

 it by the organization is to be retained with such additional abbreviation as may be 

 necessary to identify the organization. 



When a tide gage is discontinued, check levels between the tide staff and not less 

 than three bench marks shall be run to ascertain whether there has been any change in 

 the elevation of the tide staff during the observations. These levels shall be recorded 

 on Form 258, Leveling Record — Tide Station, and immediately forwarded to the Wash- 

 ington Office. 



