2. PLANS AND PREPARATIONS 



39 



cause. Search shall be made for all old bench 

 marks in the general vicinity, their present 

 condition shall be noted, and the old descrip- 

 tions revised if necessary. New bench marks 

 should be clearly described so that they may 

 be readily recovered and identified. When 

 a bench mark of another organization is re- 

 covered and connected by leveling with the 

 Coast and Geodetic Survey bench marks, the 

 number or name assigned it by the organiza- 

 tion is to be retained with such additional 

 abbreviations 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 staif during the observations. 

 These levels shall be recorded in Form 258 — 

 Leveling Record — Tide Station, and imme- 

 diately forwarded to the Washington Office. 

 Instructions for establishment of bench 

 marks and for leveling are contained in Spe- 

 cial Publication No. 196. 



2-52 Tide station reports and records. — 



Report — Tide station, Form 681, shall be 

 submitted, in duplicate, immediately for each 

 tide station established. The exact location 

 of each tide station shall be shown on each 

 hydrographic sheet (see 6-71). The time 

 meridian used shall be indicated in the tide 

 records. The tide records for portable gages 

 shall be forwarded for each two weeks of 

 satisfactory operation, if practicable. When 

 a standard gage is used the marigram should 

 be changed at the end of each calendar 

 month. When a station is discontinued, the 

 fact shall be noted on the original record. 

 The hourly heights required for reduction 

 of soundings shall be tabulated before the 

 marigrams are forwarded to the Washington 

 Office. The datum or plane of reference will 

 be derived in the office and furnished the 

 field party on request. If a primary tide 

 station is used for reduction of soundings, 

 the hourly heights for the station will be 

 furnished by the Washington Office on 

 request. 



2-53 Tide predictions. — A knowledge of 



the approximate tide is necessary in connec- 

 tion with almost all operations of a hydro- 

 graphic survey party. It is sufficient, at 

 times, to know the predicted time and 

 heights of high and low water, but for 

 some operations more exact values are nec- 

 essary. When the hydrographic survey is 

 of an area with a comparatively large 

 range of tide or where the bottom is even, 

 predicted tides must be used for the pre- 

 liminary reduction of soundings for boat 

 sheet plotting. These predictions may usu- 

 ally be obtained with sufficient accuracy 

 from the Tide Tables, but in some instances 

 a predicted tide curve may be needed and 

 will be furnished by the Washington Office, 

 on request. 



When the Tide Tables are used, the pro- 

 cedure is as follows : 



(a) From Table 2 find the tide differences 

 applicable to the area being surveyed. Apply 

 these differences to the tide predictions for 

 the reference station and obtain correspond- 

 ing times and heights of the high and low 

 waters covering the period of the work. 



(b) On cross section paper, as illustrated 

 in Fig. 4, plot the low- and high-water point 

 A and E in accordance with time and height 

 coordinates. Three cross section forms are 

 available for use depending upon the re- 

 ducer unit to be used: 114 for 0.2 foot, 115 

 for 0.5 foot, and 116 for fathoms. 



(c) Divide the connecting line AE into 

 four equal parts at points B, C, and D. 



(d) Take point B' vertically below B and 

 D' vertically above D equal to one-tenth of 

 the range of tide. 



(e) Draw an approximate sine curve 

 through points A, B', C, D', and E. This 

 curve will closely approximate the actual tide 

 curve and the required data may be readily 

 scaled from it. 



On the tide curve thus constructed, the 

 points at which changes in reducers occur 

 can be marked. A tabulation of reducers and 

 times of change can then be taken from the 

 curve and furnished each unit before starting 

 the day's work. 



2-54 Predicted tide curve. — When a pre- 



