Tidal Datums

Reference levels for tide heights, chart depths, and local water-level comparisons.

A tidal datum is a vertical reference level used to describe water height. Choosing the right datum keeps API results aligned with nautical charts, local standards, and the way your users expect tide information to be reported.

Recommended for navigation context

Use datum=CD when you want heights referenced to chart datum.

Default response

When no datum is requested, heights are referenced to mean sea level.

Computed consistently

WorldTides computes astronomical datums over a full nodal cycle of approximately 18.6 years.

Common astronomical datums

These datums are available for all locations. They are calculated over a full nodal cycle starting January 1, 2000.

AbbreviationExpansionDescription
CDChart DatumRecommended for most chart-oriented use. Chart datum is the reference level for depths on nautical charts and heights in tide tables. Request it with &datum=CD.
LATLowest Astronomical TideThe lowest predicted astronomical tide.
MLLWSMean Lower Low Water SpringsThe average of the lowest low waters during spring tide ranges.
MLWSMean Low Water SpringsThe average of successive low waters during spring tide ranges.
MHLWSMean Higher Low Water SpringsThe average of the higher low waters in successive pairs during spring tide ranges.
MLLWMean Lower Low WaterThe average of the lower low waters in successive pairs.
MLWMean Low WaterThe average of all low waters.
MHLWMean Higher Low WaterThe average of the higher low waters in successive pairs.
MLLWNMean Lower Low Water NeapsThe average of the lowest low waters during neap tide ranges.
MLWNMean Low Water NeapsThe average of successive low waters during neap tide ranges.
MHLWNMean Higher Low Water NeapsThe average of the higher low waters in successive pairs during neap tide ranges.
MTLMean Tide LevelThe average of all high and low waters.
MSLMean Sea LevelThe average sea level and the default reference level when no datum is requested.
MLHWNMean Lower High Water NeapsThe average of the lower high waters in successive pairs during neap tide ranges.
MHWNMean High Water NeapsThe average of successive high waters during neap tide ranges.
MHHWNMean Higher High Water NeapsThe average of the highest high waters during neap tide ranges.
MLHWMean Lower High WaterThe average of the lower high waters in successive pairs.
MHWMean High WaterThe average of all high waters.
MHHWMean Higher High WaterThe average of the higher high waters in successive pairs.
MLHWSMean Lower High Water SpringsThe average of the lower high waters in successive pairs during spring tide ranges.
MHWSMean High Water SpringsThe average of successive high waters during spring tide ranges.
MHHWSMean Higher High Water SpringsThe average of the highest high waters during spring tide ranges.
HATHighest Astronomical TideThe highest predicted astronomical tide.

Station-specific and regional datums

Some stations also provide fixed or regional vertical datums. These are returned only when they are available for the requested station or location.

AbbreviationExpansionDescription
NAVDNorth American Vertical DatumA fixed elevation reference determined by geodetic leveling across the United States, Canada, and Mexico.
STNDStation DatumA fixed base elevation at a tide station to which water-level measurements are referred. It is unique to each station and tied to the primary benchmark.
NN1954Norway Normal Null 1954Norwegian vertical datum of 1954.
NN2000Norway Normal Null 2000Norwegian vertical datum of 2000.