Tide Datums: Understanding Tide Heights
A tide height is the water level above or below a reference level called a datum. The datum defines zero: it is not the seabed, and it is not necessarily mean sea level.
What do the numbers mean?
A prediction of 5 ft (about 1.52 meters) means the water surface is 5 feet above the station’s stated datum. A height of −0.5 ft (about −0.15 meters) means it is half a foot below that datum. A negative tide is valid; it does not mean a negative water depth or that the sea has disappeared.
Low tide does not have to be zero. High tide and low tide describe the turning points of a tidal cycle; their heights change from day to day. The tidal range is the difference between a high tide and its adjacent low tide, measured in the same units and against the same datum.
Nautical charts show depths below chart datum, while tide heights show water levels above or below a datum. When both use the same datum and units for the appropriate location and time, adding the tide height to the charted depth gives an estimated water depth. For example, a charted depth of 10 ft plus a tide height of 3 ft gives about 13 ft of water; a tide height of −1 ft reduces it to about 9 ft. Actual depths can differ because of weather and changes to the seabed.
One water level, two different numbers
Imagine a water level reported as 6 ft above datum A. If datum B is 2 ft higher than datum A, that same water level is 4 ft above datum B. In meters, those values are approximately 1.83 m, 0.61 m, and 1.22 m, respectively. Nothing happened to the water: only the reference level changed. This is an illustrative example, not a conversion between particular datums.
Why do countries and stations use different datums?
Hydrographic authorities choose reference levels for their charts and tide predictions. Their conventions reflect local tidal conditions and surveying history. Even when two stations use the same datum name, its physical level is established locally; it is not one flat zero surface shared by every coastline.
Reference levels used by TidesPro for official-source data
This table describes the specific data sources used by TidesPro, rather than every tide product published in each country. Other products from the same authority may use a different reference level. The datum listed on an individual station’s page takes precedence.
| Country or region / source | Datum used | How to interpret it |
|---|---|---|
| United States — NOAA | MLLW | Mean Lower Low Water; the usual reference for NOAA tide predictions and U.S. nautical chart depths. |
| Mexico — SEMAR | NBMI | Nivel de Bajamar Media Inferior, a mean lower low-water reference. Use the local station reference rather than assuming an offset to U.S. MLLW. |
| Germany — BSH | SKN | Seekartennull (Chart Datum). Approximately local LAT in the German Bight’s coastal waters; tidal-river reference levels are defined separately. |
| Denmark — DMI | MSL | Mean Sea Level. The DMI API data used by TidesPro references MSL, including west-coast stations; DMI’s published chart-datum tables can show different heights. |
| Netherlands — Rijkswaterstaat | NAP | Normaal Amsterdams Peil, the Dutch national vertical reference. It is not LAT or local MSL. |
| Belgium — Meetnet Vlaamse Banken | TAW | Tweede Algemene Waterpassing, the Belgian national vertical reference used in the source tables. It is not LAT. |
| Ireland — Marine Institute | ODM | Ordnance Datum Malin, a national elevation reference. These predictions are not referenced to local chart datum. |
| Norway — Kartverket | CD | Local Chart Datum. TidesPro retains the provider’s CD label rather than assuming it is identical to LAT at every station. |
| New Zealand — LINZ | CD | Local Chart Datum, retained as published by LINZ. |
| Japan — JMA | JMA-DL | The station-specific JMA tide-table datum. The station metadata includes its published distance below MSL. |
| Taiwan — CWA | CD | Chart Datum as supplied in CWA’s tide forecast. |
| Hong Kong — HKO | CD | Hong Kong Chart Datum. |
| Costa Rica — SINAMOT | CD (provisional) | Recorded provisionally as Costa Rica Chart Datum; a more specific datum definition has not been confirmed. |
A height of 5 ft at one station cannot be compared directly with 5 ft at another to determine which water surface is higher. Even familiar labels such as CD describe a local reference, not a universal zero.
Country references: NOAA, BSH chart datum for the German Bight (PDF).
What datum do TICON-sourced stations use?
TidesPro generates these predictions from TICON-4 harmonic constants using the Slackwater prediction engine. They are derived astronomical predictions, not official government tide forecasts. TICON identifies the source of the tidal constants; it is not itself a datum.
There is no single TICON datum. TidesPro retains the reference level reported by the prediction engine for each station, using the station’s chart-datum metadata if needed. Depending on the station, labels can include MLLW, LAT, MSL, or MLWS. Do not assume a TICON station uses the official chart datum of its country, or that all TICON heights have been converted to MLLW.
The underlying datum metadata may be observation-based or derived from harmonic analysis. A calculated reference is not automatically an officially surveyed chart datum. Check the source and vertical datum on the station page before comparing heights with another tide table; converting feet to meters does not resolve a datum difference.
Common tide datums and reference levels
Some levels describe typical high or low water; others describe astronomical extremes or provide a reference for charts. They are not interchangeable.
- MLLW — Mean Lower Low Water
- The average of each tidal day’s lower low-water height over the reference period. It is not the lowest possible tide.
- MLW / MHW — Mean Low Water / Mean High Water
- Averages of all low-water heights or all high-water heights over the reference period.
- MHHW — Mean Higher High Water
- The average of each tidal day’s higher high-water height over the reference period.
- MSL — Mean Sea Level
- An average water level from observations over a stated period, rather than just the high and low tides.
- MTL — Mean Tide Level
- The midpoint between Mean High Water and Mean Low Water. It is not necessarily identical to Mean Sea Level.
- LAT / HAT — Lowest / Highest Astronomical Tide
- The lowest and highest predicted levels for the astronomical combinations considered, under average weather conditions. Weather can push actual water levels beyond these limits.
- MLWS / MHWS — Mean Low / High Water Springs
- Average low- and high-water levels during spring tides, when tidal ranges are larger. “Spring” refers to the tidal cycle, not the season.
- SKN — Seekartennull (Chart Datum)
- The German term for chart datum, shown as SKN on TidesPro’s German stations. It identifies the reference for reported heights, rather than a universal zero shared with U.S. MLLW.
- CD — Chart Datum
- The reference level for depths on a nautical chart. The abbreviation alone does not tell you which tidal definition was used; check the chart or provider’s notes.
Definitions and further reading: NOAA tidal datums · NOAA tide glossary.
Is tide height the same as water depth?
No. Tide height measures the water surface relative to a datum; depth measures the distance to the bottom. In principle, charted depth plus tide height gives water depth only when both use the same chart datum, compatible units, and the appropriate location and time. A tide height alone cannot tell you the depth at a beach, reef, or channel.
Land elevations and bridge clearances may use different reference levels again. A land-survey datum such as NAVD 88 is not automatically local mean sea level or chart datum. Converting between datums requires an established local relationship; there is no universal country-wide offset.
Reading heights on TidesPro
Look for “Tide prediction source” and “Vertical datum” near the bottom of a station’s prediction page, where that information is available. Use the station’s stated reference rather than assuming a datum from its country. If a datum is not listed, do not assume the heights are relative to mean sea level.
Changing between feet and meters changes the units, not the datum. Also check the selected date and station time zone when comparing predictions with another source.
Predicted tides describe the astronomical tide. Wind, air pressure, river flow, and other conditions can change the actual water level. TidesPro predictions are estimates and are not for navigation; use official charts, notices, and local observations for navigation decisions.