What are the main harmonic constituents?

The five principal tidal constituents are M2 (principal lunar semi-diurnal, period 12.42 h), S2 (principal solar semi-diurnal, 12.00 h), N2 (larger lunar elliptic, 12.66 h), K1 (luni-solar diurnal, 23.93 h) and O1 (principal lunar diurnal, 25.82 h). Their vector sum produces the observed tide. Ports with strong K1 and O1 relative to M2 tend toward diurnal tides; ports dominated by M2 and S2 are semi-diurnal.

What is the difference between a standard port and a secondary port?

A standard (primary) port has its own harmonic analysis; times and heights are tabulated directly for each day of the year. A secondary port is referenced to a nearby standard port using time differences and height ratios or differences listed in Volume 1 or 2 of the Admiralty Tide Tables. The correction is applied algebraically: Predicted Time/Height = Standard Port Value + Seasonal Change + Secondary Port Difference.

What datum do Admiralty Tide Tables use?

Heights in the Admiralty Tide Tables are referenced to Chart Datum (CD), which is approximately the Lowest Astronomical Tide (LAT) — the lowest level the tide is predicted to reach under average meteorological conditions. Depths on Admiralty charts are also referenced to CD, so adding the predicted tide height to a charted depth gives the total water depth at that time.

How are spring and neap ranges defined?

Spring tides occur near new and full moon when M2 and S2 are in phase; neap tides occur near first and last quarter when they oppose. The Mean High Water Springs (MHWS) and Mean Low Water Springs (MLWS) values listed in tide tables represent the average of high and low waters at spring tide. The mean spring range = MHWS − MLWS; the mean neap range = MHWN − MLWN. MHWS is used as the tidal plane on Admiralty charts for height-of-land (land features measured from MHWS, not CD).

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.

Last verified 4 months ago