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.

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.

NAVD - North American Vertical Datum of 1988 (NAVD88): A fixed reference for elevations determined by geodetic leveling. The datum was derived from a general adjustment of the first-order terrestrial leveling nets of the United States, Canada, and Mexico. In the adjustment, only the height of the primary tidal bench mark, referenced to the International Great Lakes Datum of 1985 (IGLD 85) local mean sea level height value, at Father Point, Rimouski, Quebec, Canada was held fixed, thus providing minimum constraint. NAVD 88 and IGLD 85 are identical. However, NAVD 88 bench mark values are given in Helmert orthometric height units while IGLD 85 values are in dynamic heights. See International Great Lakes Datum of 1985, National Geodetic Vertical Datum of 1929, and geopotential difference. NAVD 88 should not be used as 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.

EWL - Extreme Water Levels: The extreme levels measured by the CO-OPS tide gauges during storms are called storm tides, which are a combination of the astronomical tide, the storm surge, and limited wave setup caused by breaking waves. They do not include wave runup, the movement of water up a slope. Therefore, the 1% annual exceedance probability levels shown do not necessarily correspond to the Base Flood Elevations (BFE) defined by the Federal Emergency Management Administration (FEMA), which are the basis for the National Flood Insurance Program.

NOAA's EWLs: For this product, the monthly extreme levels adjusted for relative sea level trend, producing an unbiased data set from which the annual highest and lowest water levels are derived. A Generalized Extreme Value (GEV) analysis is applied to this data set to determine the expected frequency of extreme tides rising above or falling below any given level. The results are a set of annual exceedance probability levels relative to the tidal datums for each station.

USACE's EWLs: Produced as is the NOAA EWLs using the statistical Percentile function rather than GEV.

Linear Trend(s): The Sea Level Tracker fits a linear regression model for the monthly means when requested by the user. The user has the choice to either fit a regression to the entire selected period of record or a forty-year window. The values visible in the table and with the tooltips are the value of the fit linear model for the selected date. The linear trend rates are visible via a module that presents when the 'Trend Line Information' button below the plot is pressed.