NOAA’s National Ocean Service maintains many tidal datums. These datums are used to create accurate nautical charts so mariners can navigate safely.
Tidal datums are reference points to help measure water levels in a certain location. They are based on the average water level height during each stage in the tidal cycle. The table below contains a few examples of tidal datums maintained by NOAA’s Center for Operational Oceanographic Products and Services, or CO-OPS. To maintain these datums, CO-OPS relies on the National Tidal Datum Epoch, or NTDE — a 19-year period over which tidal data was collected to calculate average water levels. The current NTDE is from 1983 through 2001. This longer-term tidal data helps scientists standardize the data collected.
| Tidal Datum | Description |
|---|---|
| Highest Observed Tide (Max Tide) | The maximum height reached by a rising tide. |
| Lowest Observed Tide (Min Tide) | The minimum height reached by a falling tide. |
| Mean High Water (MHW) | The average of all the high water heights observed over the National Tidal Datum Epoch, or NTDE. |
| Mean Higher High Water (MHHW) | The average of the higher high water height of each day observed over the NTDE. |
| Mean Low Water (MLW) | The average of all low water heights observed over the NTDE. |
| Mean Lower Low Water (MLLW) | The average of the lower low water height of each day observed over the NTDE. |
| Mean Range of Tide (MN) | The difference in height between mean high water and mean low water. |
| Mean Sea Level (MSL) | The average of hourly water heights observed over the NTDE. |
Nautical charts rely on tide and water level datums to produce accurate, safe maps. The notes from this chart in southern Alaska indicate that depths referenced to Mean Lower Low Water, heights to Mean High Water, and other tidal information are available in the table.
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