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Instructions relating to tidal bench marks and leveling are contained in paragraphs 

 97 to 112 of Special Publication No. 196, Manual of Tide Observations, 



1434. Tide Station Reports 



A report on the establishment of each tide station shall be made in duplicate on 

 Form 681, Report — Tide Station. The record of levels, with descriptions and reports 

 on bench marks, shall be entered on Form 258, Leveling Record — Tide Station. The 

 report on the establishment of the station and the record of levels should be forwarded 

 to the Washington Office immediately after the installation of the tide station has been 

 completed. The location of each tide station and the points at which tide observations 

 are taken with the Dorsey Fathometer (see 1436) must be plotted on the hydrographic 

 sheet (see 7865). 



1435. Tide Records 



Records from the automatic tide gages are to be forwarded to the Office as promptly 

 as possible after the data required by the field party have been extracted. When the 

 observations at any tide station are termmated, a notation of the hour and date of the 

 discontinuance should be entered on the last marigram taken from the gage. 



1436. Offshore Tide Observations 



Because of the high precision of the Dorsey Fathometer (see 524), it is possible to 

 measure with it the tide in offshore areas. To obtain results of acceptable accuracy, 

 the sea should be relatively calm, the position for the observations should be selected 

 where the bottom is known to be level, and the vessel should be anchored with a 

 minimum length of chain. 



Tide observations by this method are of value for determining the characteristics 

 of the tide at a considerable distance from shore and in areas nearer shore where differ- 

 ences in range and time of tide are suspected. When such observations are correlated 

 with simultaneous tide records at shore stations, the latter may be adapted for use in 

 the reduction of offshore soundings. The observations to be of most value should be 

 continuous over at least a 25-hour period, and a considerably longer continuous series 

 is highly desirable. As a check on the accuracy of the results, repeat observations 

 should be made during different periods at the same position. 



For observing tides, echo soundings should be read at regular consecutive intervals 

 of not less than 15 minutes, recorded on Form 277, Tides, and plotted on cross-section 

 paper. Irregularities in recorded heights can then be eliminated by sketching a smooth 

 curve from the plotted values. Both recorded and adjusted heights should be included 

 in the data forwarded to the Washington Office. A report should accompany the 

 record, giving the latitude and longitude of the observations, the condition of the sea 

 (height of waves, swell, etc.), the depth of water, amount of chain used in anchoring, 

 and an interpretation of results with an estimate of accuracy obtained. When a graphic 

 recorder is used, the original fathogram should also be sent to the Office to aid in the 

 interpretation of results. 



Tide observations in offshore areas have been made with apparatus of European 

 design operated from a ship at anchor or installed on the ocean bottom and marked 

 with a buoy for future recovery. The principle of operation consists of the measurement 

 of the change of pressure as the tide rises and falls. 



A proposal has been made by the British that tides be measured by placing the 

 transmitting and receiving unit of a recording echo-sounding instrument on the sea 



