elevation, field— An elevation taken from the field computation of a line of levels. 



elevation, fixed— An elevation which has been adopted, either as a result of tide observations 

 or previous adjustment of spirit leveling, and held at its accepted value in any subsequent 

 adjustment. 



elevation, ortliometric— A preUminary elevation to which the orthometric correction has 

 been applied. 



elevation, preliminary— An elevation arrived at after tlie index, level, rod, and temperature 

 corrections liave been appUed to tlie observed differences of elevation and new elevations 

 have been computed. 



elevation, standard— An adjusted elevation based on the NGVD or the Sea Level Datum of 

 1929. 



Ellipse, eccentricity of— See eccentricity of ellipse, 



epoch— (1) Also known as phase lag. Angular retardation of the maximum of a constituent 

 of the observed tide (or tidal current) behind the corresponding maximum of the same 

 constituent of the theoretical equdibrium tide. It may also be defined as the phase 

 difference between a tidal constituent and its equilibrium argument. When referred to 

 the local equilibrium argument its symbol is the Greek letter K (kappa) ; wlien referred 

 to the corresponding Greenwich equdibrium argument it is called the Greenwich epocli 

 and is represented by the capital letter G. A Greenwich epoch that has been modified to 

 adjust to a particular time meridian for convenience in tlie prediction of tides is 

 represented by the small letter g or by k'. The relations between these epochs may be 

 expressed by 



G = K + pL 



g = k' = G - — 

 ^ 15 



where L is the longitude of the place, and S the longitude of the time meridian, these 

 being taken as positive for west longitude and negative for east longitude; p is the 

 number of constituent periods in the constituent day and is equal to for all 

 long-period constituents, 1 for diurnal constituents, 2 for semidiurnal constituents, 

 etc.; and a is the hourly speed of tlie constituent, all angular measurements being 

 expressed in degrees. (2) Used in tidal datum determinations is a 19-year metonic cycle 

 over which tidal height observations are meaned to establish the various datums. Since 

 periodic and apparent secular trends occur in sea level, a specific 19-year cycle (the 

 National Tidal Datum Epoch) is selected so that aU tidal datum determinations 

 tlirougliout the United States and its possessions will have a common reference. The 

 National Tidal Datum Epoch officially adopted by the NOS is 1941 through 1959. The 

 National Tidal Datum Epoch is scheduled for review and possible revision at 25-year 

 intervals. 



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