NOAA Tides & Currents
CO-OPS provides the national infrastructure, science, and technical expertise to monitor, assess, and distribute tide, current, water level, and other coastal oceanographic products and services that support NOAA's mission of environmental stewardship and environmental assessment and prediction. CO-OPS provides operationally sound observations and monitoring capabilities coupled with operational Nowcast Forecast modeling.
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. 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. National Geodetic Vertical Datum of 1929 (NGVD29) A fixed reference adopted as a standard geodetic datum for elevations determined by leveling. The datum was derived for surveys from a general adjustment of the first-order leveling nets of both the United States and Canada. In the adjustment, mean sea level was held fixed as observed at 21 tide stations in the United States and 5 in Canada. The year indicates the time of the general adjustment. A synonym for Sea-level Datum of 1929. The geodetic datum is fixed and does not take into account the changing stands of sea level. Because there are many variables affecting sea level, and because the geodetic datum represents a best fit over a broad area, the relationship between the geodetic datum and local mean sea level is not consistent from one location to another in either time or space. For this reason, the National Geodetic Vertical Datum should not be confused with mean sea level. See North American Vertical Datum of 1988 (NAVD 88). NGVD 29 should not be used as Mean Sea Level. NGVD 29 is no longer supported by NGS.