The average height of the high water over a 19-year period. For shorter periods of observation, corrections are applied to eliminate known variations and reduce the result to the equivalent of a mean 19-year value. All high-waters heights are included in the average where the type of tide is either semidiurnal or mixed. Only the higher high-water heights are included in the average where the type of tide is diurnal. So determined, mean high water in the latter case is the same as mean higher high water.