Source: NOAA, Center for Operational Oceanographic Products and Services
The term Mean Sea Level (MSL) can also refer to a tidal datum, or frame of vertical reference defined by a specific phase of the tide.
Tidal datums are locally-derived based on observations at a tide station, and are typically computed over a 19-year period, known as the National Tidal Datum Epoch (NTDE). The present 19-year reference period used by NOAA is the 1983-2001 NTDE.
Tidal datums must be updated at least every 20-25 years due to global sea level rise.
Tidal datums are the basis of marine boundaries, are used as a vertical reference plane in producing nautical charts, and provide important baseline information for observing changes in sea level over time. MSL as a tidal datum is computed as a mean of hourly water level heights observed over 19-years.
Monthly means are generated in the datum calculation process, which are used to generate the relative local sea level trends observed here.