XII. TIDE MEASUREMENTS 



Hydrography Section 

 Survey Branch 



A. INTRODUCTION 



The tide measuring program of the Hydrographic Office is directed 

 primarily toward obtaining tidal data for the reduction to a datum plane 

 of soundings taken at different stages of the tide and for correcting 

 levelling nets established in remote areas where a vertical datum has 

 not previously been established. These data are of interest to the sci- 

 entist, the general engineer, the mariner, and the public in general. 

 Tidal data are necessary for: the determination of the origins of 

 elevation essential to general engineering; the utilization of ports 

 and the construction of port facilities; and the prediction of tides, 

 crustal movements, and coastline changes. 



Measurements of tidal ranges are expressedin either feet or meters. 

 Accuracies of ±0.05 foot are practicable. Tides in general are caused 

 by the gravitational attractions of the moon and the sun; however, 

 variations in tidal range are complicated by the interference of irreg- 

 ularly distributed land masses with water movement, meteorological 

 conditions, and the relative positions of the sun and the moon with 

 respect to the earth. The actual effect of land mass distribution is not 

 readily apparent at any location. Although the lunar- solar effect is 

 predictable through a knowledge of the phase cycle, tidal measure- 

 ments must be made to determine the parallax cycle and the declination 

 cycle. 



Tide measuring stations are classified as primary or secondary 

 stations. Primary stations are somewhat elaborate and generally 

 are established to obtain data over long periods of time. Secondary 

 stations are established for specific purposes and are removed after 

 the requirement has been met. Errors in the datum, inherent in the 

 incomplete cycle of the secondary station, generally can be reduced 

 by comparison of the secondary station data with data obtained from 

 the nearest primary stations. 



Tide measurements generally are accomplished either by using 

 automatically registering tide gauges or nonregistering tide gauges. 

 A precise echo sounder on board an anchored ship also can be used 

 when conditions are satisfactory. The automatically registering tide 

 gauges generally consist of a float or pressure gauge, a staff, and 

 some type of clock-driven recording device, suitably housed, which 



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