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NOTES AND COMMENTS:
Mean Sea Level (MSL) is a tidal datum determined over a 19-year National Tidal Datum Epoch.  It pertains to local mean sea level and should not be confused with the fixed datums of North American Vertical Datum of 1988 (NAVD 88). The National Oceanic and Atmospheric Administration, National Ocean Service (NOS) is updating the nation's tidal datums to a new National Tidal Datum Epoch (NTDE) to reflect changes in mean sea level along the nation's coast.
The new NTDE will provide up-to-date tidal datum information available for applications that are essential to support NOAA's strategic programs, as well as other Federal, State and private sector coastal zone activities. As a result of the update, more accurate tidal information will be made available for widespread application to hydrographic surveys and mapping programs, navigational safety (Physical Oceanographic Real Time Systems, PORTS®), wetland restoration, marine boundaries, coastal engineering, storm warnings and hazard mitigation, emergency management, and hydrodynamic modeling.
The (NTDE) is a specific 19-year period over which tide observations are taken to determine Mean Sea Level and other tidal datums such as Mean Lower Low Water and Mean High Water. This latest update will define the 19-year period as 1983-2001. A tidal datum is a vertical reference based on a specific stage of tide which serves as a baseline elevation to which sounding depths or topographic heights are referenced. The 19-year period includes an 18.6 year astronomical cycle that accounts for all significant variations in the moon and sun that cause slowly varying changes in the range of tide. The NTDE has been adopted so that tidal datum determinations throughout the United States will be based on one specific common reference period.
Relative mean sea level, which is a term used to denote the average height of the ocean relative to land, can vary by location and over time and is not the same at all locations. It is the policy of NOS to consider a revised NTDE every 20-25 years in order to take into account relative sea level changes caused by global sea level change and the effects of long term local land movement due to subsidence or glacial rebound. Previous tidal Epochs were determined for periods 1924-42, 1941-59, and 1960-78. Water level measurements taken by NOS over long periods of time are used to smooth out shorter term (daily, monthly, annual) variations which help reveal long term trends (see http://tidesandcurrents.noaa.gov/sltrends/sltrends.shtml). The NOS Center for Operational Oceanographic Products and Services (CO-OPS) has the responsibility for operating and maintaining the National Water Level Observation Network, comprised of 175 long term stations in the coastal ocean and Great Lakes. The tide stations are used as the primary reference for determination of tidal datums and bench mark elevations (permanent monuments with elevations referenced to tidal datums) for station locations listed on the CO-OPS web site. A significant number of these tidal bench mark information "sheets" will be made available on the 1983-2001 NTDE after April 21, 2003. The remaining stations, that meet NOS publication standards, will be continuously updated until the process is completed at the end of FY 2003.
For further information please contact Steve Gill at 301-713-2981 Extension 139, or Stephen.Gill@noaa.gov
Boston Station Datum, PID MY0555
Data Retrieval site: http://co-ops.nos.noaa.gov/waterlevels.html?id=8443970
Datum site: http://co-ops.nos.noaa.gov/datums.html?units=1&epoch=0&id=8443970&name=B...
ELEVATIONS ON STATION DATUM,
National Ocean Service (NOAA)
Station:  8443970                                           T.M.:         0 W
Name:     Boston, MA                                       Units:     Meters
Status:   Accepted (Apr 17 2003)                   Epoch:  1983-2001
Datum:       STND
Datum         Value     Description
---------  --------  ----------------------------------------
MHHW          4.205    Mean Higher-High Water
MHW             4.071   Mean High Water
NAVD88       2.752   North American Vertical Datum of 1988
MSL              2.660   Mean Sea Level
DTL              2.640    Mean Diurnal Tide Level
MTL              2.624   Mean Tide Level
MLW             1.178   Mean Low Water
MLLW           1.074   Mean Lower-Low Water
STND            0.000   Station Datum