geoid, isostatic— An ideal geoid derived from tlie spheroid of reference by the application of 

 computed values of the deflection of tlie vertical which depend upon the topography and 

 isostatic compensation. The computed values of tlie deflection of the vertical used in 

 obtaining the isostatic geoid are similar to those for the compensated geoid, but of 

 opposite signs. If the theory and assumptions with regard to isostasy were exact and 

 there were no anomalies, the isostatic geoid would agree with the actual geoid. 



geoid contour— A line on the surface of the geoid of constant elevation with reference to the 

 surface of the spheroid of reference. Geoid contours represent differences in elevation 

 between the geoid and the spheroid. Geoid contours depend on the surface of reference. 

 The same geoid referred to Afferent surfaces of reference will give different sets of geoid 

 contours. 



glacioisostatic adjustment-The surface of the Earth was depressed due to the weight of the 

 ice during the ice ages. A slow recovery is stiU in progress, leading to increasing land 

 levels in the areas formally covered by ice. This slow recovery is called a glacioisostatic 

 adjustment. 



great diurnal range (Gt)— The difference in height between mean higher high water and mean 

 lower low water. The expression may also be used in its contracted form, diurnal range. 



gulf coast low water datum— A reference datum, defined as being equal to the low water 

 datum for diurnal tides, and to the lower low water datum for mixed or semidiurnal 

 tides; used only for the Gulf of Mexico and southern Florida, southwest of Key Largo. 



half-tide level-Also called mean tide level (MTL). A tidal datum midway between mean 

 higli water and mean low water. 



harmonic analysis— The process by which the observed tide or tidal current at any place is 

 separated into basic harmonic constituents. The process is described in detail in Manual 

 of Harmonic Analysis and Prediction of Tides (U.S. Coast and Geodetic Survey, 1941). 



harmonic constants— The amplitudes and epochs of the harmonic constituents of the tide or 

 tidal current at any place. 



harmonic constituent— See constituent. 



high water (HW)-The maximum height reached by a rising tide. The height may be due 

 solely to the periodic tidal forces or it may have superimposed upon it the effects of 

 prevailing meteorological conditions. Use of tlie synonymous term, high tide, is 

 discouraged. 



high water mark— A Une or mark left on tidal flats, beach, or alongshore objects indicating 

 the elevation of the intrusion of high water. The mark may be a line of oil or scum on 

 alongshore objects, or a generally continuous deposit of fine shell or debris on the 

 foreshore or berm. This mark is physical evidence of the general height reached by wave 

 runup at recent high waters. It should not be confused with the mean high water Une or 

 mean higher high water line. 



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