A tidal datum is height of the water used as a zero reference. In some areas, tidal datums are calculated from a full nodal cycle (~18.6 year period) of high and low waters. In other areas, datums are chosen based on historical significance.

Definitions

The following datums will be returned for all locations. These datums are calculated over a full nodal cycle (~18.6 years) starting at Jan 1st, 2000.

Abbreviation	Expansion	Description
CD	Chart Datum (Recommended)	Reference level for depths in navigational charts and heights in tide tables. The actual level depends on local customs. For historical reasons, this is not the default. However, we recommend most users use this by sending the &datum=CD option.
LAT	Lowest Astronomical Tide	The lowest predicted astronomical tide.
MLLWS	Mean Lower Low Water Spring	The average of the lowest tide during spring tide range.
MLWS	Mean Low Water Spring	The average of the successive pair of lowest tides during spring tide range.
MHLWS	Mean Higher Low Water Spring	The average of the higher of the successive pair of lowest tides during spring tide range.
MLLW	Mean Lower Low Water	The average of the lowest of the successive pair of low tides.
MLW	Mean Low Water	The average of all low waters
MHLW	Mean Higher Low Water	The average of the highest of the successive pair of low tides.
MLLWN	Mean Lower Low Water Neaps	The average of the lowest tide during neap tide range.
MLWN	Mean Low Water Neaps	The average of the successive pair of lowest tides during neap tide range.
MHLWN	Mean Higher Low Water Neaps	The average of the higher of the successive pair of lowest tides during neap tide range.
MTL	Mean Tide Level	The average of all high and low tides.
MSL	Mean Sea Level (Default)	The average sea level.
MLHWN	Mean Lower High Water Neaps	The average of the lower of the successive pair of highest tides during neap tide range.
MHWN	Mean High Water Neaps	The average of the successive pair of highest tides during neap tide range.
MHHWN	Mean Higher High Water Neaps	The average of the highest tide during neap tide range.
MLHW	Mean Lower High Water	The average of the lowest of the successive pair of high tides.
MHW	Mean High Water	The average of all high waters
MHHW	Mean Higher High Water	The average of the highest of the successive pair of high tides.
MLHWS	Mean Lower High Water Spring	The average of the lower of the successive pair of highest tides during spring tide range.
MHWS	Mean High Water Spring	The average of the successive pair of highest tides during spring tide range.
MHHWS	Mean Higher High Water Spring	The average of the highest tide during spring tide range.
HAT	Highest Astronomical Tide	The highest predicted astronomical tide.

The following datums will only be returned for only some stations:

Abbreviation	Expansion	Description
NAVD	North American Vertical Datum (US)	A fixed reference for elevations determined by geodetic leveling. The datum is derived from a general adjustment of the first-order terrestrial leveling nets of the United States, Canada, and Mexico.
STND	Station Datum (US)	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.
NN1954	Norway Normal Null 1954 (Norwegian)	Norwegian vertical datum of 1954.
NN2000	Norway Normal Null 2000 (Norwegian)	Norwegian vertical datum of 2000.