Tide tables

Tide tables predict the height and time of high and low water at standard ports and secondary ports around the world. Published by national hydrographic offices — UKHO (Admiralty Tide Tables, 4 volumes) and NOAA (US) — they are derived from harmonic analysis of observed water levels. The navigator must apply secondary-port corrections, datum offsets, and meteorological adjustments before use.

Harmonic constituents

• · M2 (12.42 h) is the dominant constituent in most ports; its amplitude largely determines the tidal range.
• · S2 (12.00 h) beats against M2 to produce the spring-neap cycle with a period of 14.77 days (half a synodic month).
• · N2 (12.66 h) introduces inequality between successive semi-diurnal tides depending on the moon's elliptical orbit.
• · K1 and O1 generate diurnal inequality; where they dominate over M2 a port may have only one high water per day.
• · Over 60 constituents are used in a full harmonic analysis; IHO Publication M-3 defines the naming and coding standard.

Standard vs secondary ports

• · Standard port predictions are tabulated directly for every high/low water in the year.
• · Secondary port corrections comprise a time difference (±hours/minutes) and either a height difference (±metres) or a height ratio applied to the standard port value.
• · Where height differences vary with the range (springs vs neaps), interpolation between MHWS and MLWS values is required using the seasonal correction curve supplied in the tide tables.
• · Always check whether the time difference is referenced to UT (UTC) or local time — standard ports list times in UT.

Height and time interpolation

• · To find the height at an intermediate time, the Rule of Twelfths or the tidal curve (Port Information Book) can be used.
• · Rule of Twelfths: rise/fall by 1/12 of range in hour 1, 2/12 in hour 2, 3/12 in hours 3 and 4, 2/12 in hour 5, 1/12 in hour 6.
• · Admiralty Tide Tables include port mean spring/neap curves for more accurate interpolation using Factor F = (range − neap range)/(spring range − neap range).
• · To find the time at which a given height will be reached, reverse the interpolation on the tidal curve.

Datums (CD, MSL, LAT, MHWS)

• · Chart Datum (CD): reference for depths on charts and for tide table heights; approximately equal to LAT.
• · Lowest Astronomical Tide (LAT): the lowest level predicted under any combination of astronomical conditions.
• · Mean Sea Level (MSL): mean of all observed water levels; approximately mid-way between MHWS and MLWS.
• · MHWS is used as the datum for heights of land features (bridges, cliffs) on Admiralty charts.
• · Under-keel clearance (UKC) = charted depth + predicted tide height − ship's draught; always use the correct datum.

Meteorological and storm surge corrections

• · Tide tables give astronomical predictions only; actual water levels may differ by ±0.5 m or more in shallow-water ports.
• · Onshore winds and low barometric pressure raise water levels (storm surge); offshore winds and high pressure lower them.
• · The inverted barometer effect: a 1 hPa drop in pressure raises sea level approximately 1 cm.
• · Real-time observations from tide gauges (NTSLF, NOAA CO-OPS) should be checked before critical shallow-water transits.

Sources: UKHO Admiralty Tide Tables NP201-NP204 (annual volumes); NOAA Tide Tables (tidesandcurrents.noaa.gov); IHO Publication M-3 — Resolutions of the IHB on Tidal and Water Level Datums. See also chart datum, under-keel clearance, passage planning.