value. Use of the synonymous term, mean high tide, is discouraged. For a semidiurnal or 

 mixed tide, tlie two liigh waters of each tidal day are included in tixe mean. When any 

 lower liigh water is indistinct, it is determined by record examination. For a diurnal tide, 

 the one high water of each tidal day is used in tlie mean. In the event a second high water 

 occurs, only tlie diurnal high water is included (see diurnal). So determined, tliis mean 

 high water, based on the diurnal tide, is the equivalent of mean higlier high water of a 

 mixed tide. See datum and type of tide. 



mean high water line (MHWL)— The intersection of the land with the water surface at tlie 

 elevation of mean high water. See shoreline. 



mean higher high water (MHHW)— A tidal datum. The arithmetic mean of tlie higher high 

 water heights of a mixed tide observed over a specific 19-year metonic cycle (the 

 National Tidal Datum Epoch). See epoch (2). Only tlie higher high water of each pair of 

 high waters of a tidal day is included in tlie mean. For stations witli sliorter series, 

 simultaneous observational comparisons are made with a primary control tide station to 

 derive the equivalent of a 19-year value. See datum and type of tide. 



mean higher high water line (MHHWL)— The intersection of the land with the water surface 

 at the elevation of mean higher high water. 



mean low water (MLW)— A tidal datum. The aritlimetic mean of the low water heights 

 observed over a specific 19-year metonic cycle (the National Tidal Datum Epoch). See 

 epoch (2). For stations with shorter series, simultaneous observational comparisons are 

 made with a primary control tide station to derive the equivalent of a 19-year value. Use 

 of the synonymous term, mean low tide, is discouraged. For a semidiurnal or mixed tide, 

 the two low waters of each tidal day are included in the mean. When any higher low 

 water is indistinct, it is determined by record examination. For a diurnal tide, tlie one 

 low water of each tidal day is used in the mean. In the event a second low water occurs, 

 only the diurnal low water is included (see diurnal). So determined, this mean low water, 

 based on the diurnal tide, is the equivalent of mean lower low water of a mixed tide. See 

 datum and type of tide. 



mean low water line (MLWL)— The intersection of the land with the water surface at tlie 

 elevation of mean low water. 



mean low water springs (MLWS)— A tidal datum. See datum. Frequently abbreviated spring 

 low water. The arithmetic mean of the low water heights occurring at the time of tlie 

 spring tides observed over a specific 19-year metonic cycle (the National Tidal Datum 

 Epoch). It is usually derived by taking an elevation depressed below the half-tide level by 

 an amount equal to one-half of the spring range of tide, with necessary corrections 

 apphed to reduce the result to a mean value. This datum is used, to a considerable 

 extent, for hydrographic work outside of the United States and is the level of reference 

 for the Pacific approaches to the Panama Canal. 



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