1. Understanding Tidal Datums
Dr. Patrick R. Michaud
Conrad Blucher Institute
Division of Nearshore Research
February 9, 2001

2. Presentation Overview
Definitions
Understanding Tidal Datums
Computing Tidal Datums
Other Notes

4. Tidal Datum Definitions
Tide – The periodic rise and fall of the water resulting from gravitational interactions between Sun, Moon, and Earth.
High water – The maximum height reached by a rising tide.
Low water – The minimum height reached by a falling tide.

5. Tidal Datum Definitions
Higher High Water – The highest of the high waters (or single high water) of any specified tidal day due to the declinational effects of the Moon and Sun
Lower Low Water – The lowest of the low waters (or single low water) of any specified tidal day due to the declinational effects of the Moon and Sun

6. Example Tides

7. Tidal Datum Definitions
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 for tidal datums.

8. MHHW, MHW
Mean Higher High Water (MHHW) – The average of the higher high water height of each tidal day observed over the National Tidal Datum Epoch.
Mean High Water (MHW) – The average of all of the high water heights of each tidal day observed over the NTDE.

9. MLW, MLLW
Mean Low Water (MLW) – The average of all of the low water heights of each tidal day observed over the NTDE.
Mean Lower Low Water (MLLW) – The average of the lower low water height of each tidal day observed over the NTDE.

11. MTL, MSL
Mean Tide Level (MTL) – The arithmetic mean of mean high water and mean low water.
Mean Sea Level (MSL) – The arithmetic mean of hourly heights observed over the NTDE.

16. Datums relative to NAVD88

17. Datums relative to MSL

18. Datums relative to MLLW