HAT
Highest Astronomical Tide | The elevation of the highest predicted astronomical tide expected to occur at a specific tide station over the time period of 40 years. The 40 years period will include 2 National Tidal Datum Epoch periods. This time period will be updated every 20 years. The present values are based on the time period of 2000-2040. |
MHHW*
Mean Higher High Water | The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch. |
MHW
Mean High Water | The average of all the high water heights observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch. |
DTL
Diurnal Tide Level | The arithmetic mean of mean higher high water and mean lower low water. |
MTL
Mean Tide Level | The arithmetic mean of mean high water and mean low water. |
MSL
Mean Sea Level | The arithmetic mean of hourly heights observed over the National Tidal Datum Epoch. Shorter series are specified in the name; e.g. monthly mean sea level and yearly mean sea level. |
MLW
Mean Low Water | The average of all the low water heights observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch. |
MLLW*
Mean Lower Low Water | The average of the lower low water height of each tidal day observed over the National Tidal Datum Epoch. For stations with shorter series, comparison of simultaneous observations with a control tide station is made in order to derive the equivalent datum of the National Tidal Datum Epoch. |
LAT
Lowest Astronomical Tide | The elevation of the lowest predicted astronomical tide expected to occur at a specific tide station over the time period of 40 years. The 40 years period will include 2 National Tidal Datum Epoch periods. This time period will be updated every 20 years. The present values are based on the time period of 2000-2040. |
GT
Great Diurnal Range | The difference in height between mean higher high water and mean lower low water. |
MN
Mean Range of Tide | The difference in height between mean high water and mean low water. |
DHQ
Mean Diurnal High Water Inequality | One-half the average difference between the two high waters of each tidal day observed over the National Tidal Datum Epoch. It is obtained by subtracting the mean of all the high waters from the mean of the higher high waters. |
DLQ
Mean Diurnal Low Water Inequality | One-half the average difference between the two low waters of each tidal day observed over the National Tidal Datum Epoch. It is obtained by subtracting the mean of the lower low waters from the mean of all the low waters. |
HWI
Greenwich High Water Interval | The average interval (in hours) between the moon's transit over the Greenwich meridian and the following high water at a location. |
LWI
Greenwich Low Water Interval | The average interval (in hours) between the moon's transit over the Greenwich meridian and the following low water at a location. |
Max Tide
Highest Observed Tide | The maximum height reached by a rising tide. The high water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions. |
Min Tide
Lowest Observed Tide | The minimum height reached by a falling tide. The low water is due to the periodic tidal forces and the effects of meteorological, hydrologic, and/or oceanographic conditions. |
Station Datum | A fixed base elevation at a tide station to which all water level measurements are referred. The datum is unique to each station and is established at a lower elevation than the water is ever expected to reach. It is referenced to the primary bench mark at the station and is held constant regardless of changes to the water level gauge or tide staff. The datum of tabulation is most often at the zero of the first tide staff installed. |
National Tidal Datum Epoch | The specific 19-year period adopted by the National Ocean Service as the official time segment over which tide observations are taken and reduced to obtain mean values (e.g., mean lower low water, etc.) for tidal datums. It is necessary for standardization because of periodic and apparent secular trends in sea level. The present NTDE is 1983 through 2001 and is actively considered for revision every 20-25 years. Tidal datums in certain regions with anomalous sea level changes (Alaska, Gulf of Mexico) are calculated on a Modified 5-Year Epoch. |