Glossary of Terms Used in Tidal Measurements and Analysis

Extended from Volume 1 of the IOC Manual and to be used in combination with Volume 3.

See also Pugh (1987) for a glossary of other terms used in tidal science.

Analysis (Tidal or Harmonic)

A tidal analysis is normally a linear regression of a sea level time series (of, for example, hourly values) in terms of harmonic tidal constituents (sine waves). The analysis results in 'tidal harmonic constants' (amplitudes and phase lags for each tidal constituent). There are also tidal analyses based on the response method (see Pugh, 1987 for a fuller description of tidal analyses). For the purpose of a computer tidal analysis, the observed sea levels are separated into three components:-

(1) Mean sea level
(2) Tidal levels
(3) Surge (or residual) levels

However, note that in many other applications the MSL and surge together are best considered as the 'non-tidal' term.

Chart Datum

Chart datum is the low water plane, below which depths on a nautical chart are measured and above which tidal levels are sometimes measured.

Contact Mark (or Contact Point)

A permanent reference mark at a tide gauge from which water level measured by the gauge can be directly related. The Contact Mark is also the main reference mark on the gauge hardware to be related to benchmarks in the surrounding area by geodetic levelling (see Volume 3 of the IOC Manual). This mark will be located either on the container of the gauge (e.g. for an acoustic gauge) or on some stable object near the stilling well (for a float gauge).

Data Reduction

The process of checking, calibration, and preparation necessary to convert raw measurements into a form suitable for analysis and application.


Geodetic vertical reference levels. See also Volume 3 of the IOC Manual for a list of often-used datums.

Diurnal Tides

Tides having periods of approximately 1 day.

Extreme High or Low Water

The highest or lowest elevation reached by the sea during a given period.

Harbour Datum

Harbour datum is a horizontal plane, defined by the local harbour authority, from which levels and tidal heights are measured by that authority.

Harmonic Constants

Tides are periodic oscillations generated by and related to the motions and attractive forces of the moon, sun and earth system. The tide can be represented by the sum of a series of sine waves of determined frequency "harmonic constituents". The parameters of each sine wave are called "harmonic constants", and are the amplitude (half the height) of the wave and phase, or time of occurrence, of the maximum.

Higher High Water (H.H.W.)

The higher of the high waters of any specified day. In a semidiurnal tidal regime there will be two high tides a day, and in a diurnal regime there may be just the one (or one main high tide and a minor one).

Higher Low Water (H.L.W.)

The higher of the low waters of any specified day.

Highest and Lowest Astronomical Tide (H.A.T. & L.A.T. )

The highest and lowest levels respectively-which can be predicted under average meteorological conditions. These are not the extreme levels which can be reached as storm surges may cause considerably higher and lower levels to occur.

Inlet (of a Stilling Well Float Gauge)

The small hole or holes situated at or near the bottom of the stilling well through which the tidal water passes in and out. The area and form of the inlet are determined in relation to the area of cross section of the well, so as to achieve maximum damping of unwanted oscillations without causing an acceptable departure from the height of the tide to be recorded.

Indian Spring Low Water (I.S.L.W.)

A datum originated by Darwin when investigating tides of India. It is an elevation depressed below mean sea level by an amount equal to the sum of the amplitudes of the harmonic constituents M2,S2,K1 and O1.

Long Period Tides

Tides having periods longer than the normal semidiurnal and diurnal tides (e.g. with periods of 1 year, 6 months, 1 month or a fortnight). For a more precise definition, including a description of the latitude dependence of long period tides, see Pugh 1987.

Lower High Water (L.H.W.)

The lower of the high waters of any specified day.

Lower Low Water (L.L.W.)

The lower of the low waters of any specified day.

Mean Higher High Water (M.H.H.W.)

The average of all the higher high water heights over a period.

Mean High Water (M.H.W.)

The average of all high water heights observed over a period.

Mean High Water Neaps (M.H.W.N.)

The average of high water heights occurring at the time of neap tides.

Mean High Water Springs (M.H.W.S.)

The average of high water heights occurring at the time of spring tides.

Mean Lower Low Water (M.L.L.W.)

The average of the lower low water heights over a period.

Mean Low Water (M.L.W.)

The average of all low water heights observed over a period.

Mean Low Water Neaps (M.L.W.N.)

The average of the low water heights occurring at the time of neap tides.

Mean Low Water Springs (M.L.W.S.)

The average of the low water heights occurring at the time of spring tides.

Mean Sea Level (M.S.L.)

The mean value of sea level extracted from a suitably long series of data (e.g. a month).

Mean Tidal Range (M.T.R.)

The difference between MHW and MLW.

Mean Tide Level (M.T.L.)

The arithmetic mean of mean high water and mean low water over a suitably long period (e.g. a month).

Mixed Tide

Type of tide with a large inequality in either the high and/or low water heights.

National Datum

A fixed reference adopted as a standard geodetic datum for elevations determined by levelling.

Neap Tide

Tides of decreased range occurring semimonthly as the result of the Moon being in the first or third quarter.

Non-Linear Tides

Tides produced in shallow water or by frictional effects which have periods equivalent to 4, 6, 8 cycles or more per day.

Permanent Tide Scale (or Tide Board)

A fixed, immersed scale on which the level of the tide can be directly read. A permanent scale for use with tide gauges should be set up in open tidal water close to the tide gauge installation.

(1) with its zero mark in the plane of the tide gauge datum, and

(2) so that the scale can be accurately read from the site of the recorder.

A permanent tide scale may be a pole, board or graduations on a wall.


An electrode designed to sense water level by contact, usually in a stilling well. In some installations a probe is set at a pre-determined height to produce a mark on the recorder graph as the water level reaches it. Portable probes are also used to check the level of the water in the well.

Radiational Tides

These are tidal variations caused by solar radiation. Like the gravitational tides, but unlike meteorological effects, they are coherent in time.

Range (Tidal)

The difference between high and low water in a tidal cycle. Ranges greater than 4 m are sometimes termed macrotidal and those less than 2 m are termed microtidal. Intermediate tanges are termed mesotidal.


That part of an automatic tide gauge that records the height and time of tide. The record is nowadays usually an electronic data logger or interface to a remote computer. In older gauges it would have been a chart recorder to make a graph on specially printed paper or a punched paper or magnetic tape recorder. The recorder may be close by the monitoring point or remote from it.

Response Analysis

The representation of observed tidal variations in terms of the frequency-dependent amplitude and phase responses to input or forcing functions, usually the gravitational potential due to the Moon and Sun, and the radiational meteorological forcing.

Return Period

The average time between events such as the flooding of a particular level. This information may also be expressed as the level which has a particular return period of flodding, for example, 100 years. The inverse of the return period is the statistical probability of an event occurring in any individual year.

Sea Level

The observed level of the sea surface relative to a predefined datum at any instant of time.

Seasonal Modulations

These are variations in sea level with time scales of 1 year and harmonics of a year (6 months etc.). Part of the seasonal modulation is due to long period tides of gravitational origin (i.e. due to the Sun and Moon) and part will be due to seasonal forcings from regional/local meteorology and oceanography (e.g. seasonal steric changes or wind forcing). The seasonal tidal harmonics determined in a tidal analysis will the combination of these terms.

Secular Trends (of MSL)

The non-periodic tendency of sea level to rise, fall, and/or remain stationary with time. For example, secular trends of MSL are usually measured over many decades or a century with rates of change of the order of mm/year.


A seiche is a short-period oscillation occurring in a harbour, bay, or gulf, analogous to the oscillations of water sloshing in a bath. They are a phenomenon of local resonance and are unrelated to tides.

Semi-Diurnal Tides

Tides having periods of approximately 12 hours.

Spring Tide

Tides of increased range occurring semimonthly as the result of the Moon being new or full.

Stilling Well

A tube in which a float travels up and down according to the vertical movement of sea level.

Surge Level

The meteorologically induced component, sometimes called the ‘non tidal residual’. A large (positive) surge caused by an extreme meteorological event is called a 'storm surge'. Surge levels are not predictable in a time series as are tidal levels but their statistics do have some regularity (e.g. surges in Europe tend to persist for 1-2 days).

Tidal Level

That part of the observed sea level which is coherent with tide generating forces and is therefore predictable from a set of harmonic constants.

Tide Gauge (or Sea Level Recorder)

Apparatus for determining the level of the sea.

Tide Gauge Bench Mark

A stable bench mark near a gauge to which tide gauge datum is referred. It is connected to local auxiliary marks to check local stability and to guard against accidental damage.

Tide Gauge Datum

The horizontal plane, defined at a fixed level below a tide gauge benchmark, from which tidal heights are measured at a tide gauge. It often coincides with Chart Datum or Harbour Datum. See Volume 3 of the IOC Manual for more on datums.


A shallow water progressive wave, potentially catastrophic, caused by underwater earthquake or volcano.