Definitions for content on tides and currents

Browse the following glossary for the meaning of terms, expressions and acronyms that are used in content for tides and currents.

This list was compiled from the:

  • Canadian Tidal Manual produced by the Canadian Hydrographic Service
  • hydrographic dictionary created by the International Hydrographic Organization

Select the first letter of the term or expression you’re looking for.



Atlantic Daylight Standard Time
Amphidromic point
A no-tide point, from which cotidal lines radiate.

In physics: the maximum departure of a wave or other periodic phenomenon from the average or 0 position.

In tide terminology: the semi-range of the harmonic constituent.

The orbital point farthest from the sun when the sun is the centre of attraction. The opposite of perihelion.
Apogean tides or tidal currents
Tides of decreased range or currents of decreased speed. They occur monthly when the moon is near apogee.
That orbital point farthest from the earth when the earth is the centre of attraction. The opposite of the perigee.
Atlantic Standard Time, UTC-4



A permanent, stable object containing a marked point of known elevation with respect to a datum. It’s used as a reference level for tidal observations or as a control point for levelling.

A tidal benchmark is near a tide station to which the tide staff and tidal datums are referred.

A primary benchmark is the principal (or only) mark of a group of tidal benchmarks to which the tide staff and tidal datums are referred. The standard tidal benchmark of the Canadian Hydrographic Service is an alloy disk made of:

  • brass
  • bronze
  • aluminum
Bore (Tidal Bore)

A high breaking wave of water advancing rapidly up an estuary.

Bores can occur at the mouths of shallow rivers if the tide range at the mouth is large. They can also be generated in a river when tsunamis enter shallow coastal water and propagate up the river.

Also called:

  • eager
  • mascaret
  • tidal bore
Moncton Bore

Moncton Bore


Chart datum
Central Daylight Standard Time
Chart datum 2000
Canadian Geodetic Vertical Datum 1928 adjustment
Chart datum (CD)

Chart datum is the plane of vertical reference to which all charted depths and drying heights are related.

In non-tidal waters, it’s also the vertical datum for elevations and clearances. It’s chosen to show the least depth of water found in any place under ‘normal’ meteorological conditions. It will be a plane so low that the water level will seldom fall below it.

The surface of CD will vary from place to place with the:

  • range of tide
  • slope of the river at low stage (in non-tidal waters)

In non-tidal lakes, CD is normally a single level surface over the whole lake.

Classification of tides

Tides are frequently classified according to the diurnal inequality they display. This is a means of providing a simple description of character of the tide in various regions.

The formal classification is usually made on the basis of the ratio of some combination of the diurnal harmonic constituents over a combination of the semidiurnal constituents. The purpose of defining a ratio is to automate the classification once the constituents are known, avoiding the need to scan long periods of record visually.

Regardless of the method used, the intent is to classify tides into 4 groups, qualitatively described as follows:

  1. semidiurnal (SD): 2 nearly equal high waters and 2 nearly equal low waters approximately uniformly spaced over each lunar day
  2. mixed, mainly semidiurnal (MSD): 2 high waters and 2 low waters each lunar day, but with marked inequalities in height and irregularities in spacing
  3. mixed, mainly diurnal (MD): sometimes 2 unequal high waters and low waters at irregular spacing over a lunar day, and sometimes only 1 high water and 1 low water in a day
  4. diurnal (D): only 1 high water and 1 low water each lunar day
Coriolis force
Composite centrifugal force due to the rotation of the earth. It acts on moving particles whose motion is considered relative to that of the earth.
Cotidal line
A line on chart passing through all points where high water occurs at the same time.
Counter current
A secondary current setting in a direction opposite to that of the main current.
The highest part of a wave, swell, ridge, etc.
Central Standard Time, UTC-6

Generally, a horizontal movement of water. Currents may be classified as tidal and non-tidal.

Tidal currents are:

  • caused by gravitational interactions between the sun, moon, and earth
  • part of the same general movement of the sea that’s manifested in the vertical rise and fall (called tide)
  • periodic with a net velocity of 0 over the particular tidal cycle

Non-tidal currents include:

  • the permanent currents in the general circulatory systems of the sea
  • temporary currents arising from more pronounced meteorological variability
Current meter

An instrument for measuring the speed and direction, or just the speed, of a current. Current meters can be:

  • electric
  • acoustic
  • mechanical
  • electromagnetic
  • combination
Current station

The geographic location at which current observations are conducted. These may include a:

  • buoy
  • ground tackle
  • current meters
  • radio transmitter
  • recording mechanism



Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities.

In geodesy, a datum is defined by a set of parameters specifying the reference surface or the reference coordinate system. Therefore 2 types of datums are required:

  1. horizontal datum, which forms the basis for computations of horizontal control surveys in which the curvature of the earth is considered
  2. vertical datum, to which elevations are referred

Differences are the adjustments applied to the predictions at a reference port or reference current station. These adjustments are used to obtain predictions at a secondary port or secondary current station.

Diurnal inequality

Diurnal inequality is the difference of each tidal day in:

  • height of the 2 high waters, or of the 2 low waters or
  • speed between the 2 flood tidal currents, or the 2 ebb currents

Diurnal inequality is sometimes called declinational inequality.

Department of Public Works
Drift (current)
A wide, slow-moving ocean current principally caused by winds.
Duration of flood and duration of ebb

Duration of flood is the interval of time in which a tidal current is flooding. The duration of ebb is the interval in which a tidal is ebbing.

The intervals are reckoned from the middle of the intervening slack waters or minimum currents. Together, they cover, on average, a period of 12.42 hours for a semidiurnal tidal current or a period of 24.84 hours for a diurnal current.

In a normal semidiurnal tidal current, the duration of flood and duration of ebb will each be approximately equal to 6.21 hours. These times may be modified greatly by the presence of a non-tidal flow.

In a river, the duration of ebb is usually longer than the duration of flood because of the fresh water discharge. This is especially true during the spring months when snow and ice melt are the predominant influences.

Dynamic Vertical Clearance

Instantaneous vertical clearance between the water’s surface and an overhead structure, such as a bridge or power line. Determined either by direct air gap observation, or by calculation based on the static clearance and predicted, real-time observed, or forecast water levels.


Ebb current (ebb)

The horizontal movement of water associated with the falling tide.

Ebb currents generally set seaward, or in the opposite direction to the progression.

Also called:

  • ebb
  • ebb current
  • outgoing stream
Ebb tide (tide: falling)
The portion of the tide cycle between high water and the following low water. The opposite of rising tide.
The great circle formed by the intersection of the plane of the earth's orbit around the sun (or apparent orbit of the sun around the earth) and the celestial sphere.

A circular movement of water usually formed where currents pass obstructions:

  • between 2 adjacent currents flowing counter to each other or
  • along the edge of a permanent current
Edge waves
An ocean wave travelling parallel to the coast, with crests normal to the coastline. Such a wave has a height that diminishes rapidly seaward and is negligible at a distance of 1 wavelength offshore.
Eastern Daylight Standard Time

The 2 points in the celestial sphere where the celestial equator intersects the ecliptic. Also, the times when the sun crosses the equator at these points.

The vernal equinox is the point where the sun crosses the equator from south to north, and it occurs about March 21. Celestial longitude is reckoned eastward from the vernal equinox. The autumnal equinox is the point where the sun crosses the equator from north to south, and it occurs about September 23.

Eastern Standard Time, UTC-5


Flood current (stream)

The horizontal movement of water associated with the rising tide. Flood streams generally set toward the shore or in the direction of the tide progression.

Also called:

  • flood
  • flood current
  • in-going stream
Flood tide (Rising)
The portion of the tide cycle between low water and the following high water. The opposite of falling tide. Also called flood tide.
Projected future water levels, including approximation of all sources of water level variability. The calculation is based on the combination of tidal predictions and recently observed differences between tidal predictions and real-time observations. Only for operational stations providing real-time observations.


Geodetic datum
Geodetic datum

A set of parameters specifying the reference surface or the reference coordinate system used for geodetic control in the calculation of coordinates of points on the earth.

Commonly, datums are defined as horizontal and vertical datums separately.

For a local geodetic datum, the reference surface is defined by the:

  • latitude and longitude of an initial point
  • azimuth of a line from this point
  • parameters of the reference spheroid

Absolute datums specify the initial point of the reference ellipsoid to be (ideally) located at the earth's centre of mass.

For modern reference systems, using datum information given by satellites, additional parameters are defined such as gravity models.

Geodetic Survey of Canada
Greenland Standard Time
Greenwich Mean Time


Harmonic analysis of tide

The mathematical process by which the observed tide at a place is analyzed. It’s broken down into a number of constituent tides of simple periodic forces, each having a fixed period.

In this process, the sun and moon are replaced by a number of hypothetical tide-producing bodies which move in circular orbits around the earth in the plane of the equator. See harmonic constants, harmonic constituent.

Harmonic constants
The amplitudes and epochs of the harmonic constituents of the tide, or tidal current at any place.
Harmonic constituent

A harmonic element in a mathematical expression for the tide-producing force, and in corresponding formulae for the tide or tidal current. Each constituent represents a periodic change or variation in the relative positions of the earth, sun and moon.

Also called tidal constituent or component.

Harmonic prediction

In tidal terminology, the method of predicting tides and tidal currents by combining the harmonic constituents into a single curve.

The work was usually done mechanically by a machine designed for this purpose, called a tide predicting machine. Nowadays the work is done by computers.

Height of tide

Height of tide is the vertical distance between the surface of the sea and chart datum.

The total depth of water is found by adding the height of tide to the charted depth. For example, at a place where the chart shows 6 m (19.7 ft.) and the predicted low water height is 1 m (3.3 ft.), then the actual depth over the seabed at low water will be 7 m (23 ft.).

In the case of some ports which aren’t navigable at low water and where vessels rest on keel blocks or mattresses during low tide, the heights of the tide are measured from those keel blocks or mattresses.

High tide (HW)
The highest level reached at a place by the water surface in 1 oscillation. Also called high water.
Higher high water large tide (HHWLT)
The average of the highest high waters, 1 from each of 19 years of predictions.
Higher high water mean tide (HHWMT)
The average from all the higher high waters from 19 years of predictions.
Highest Recorded Water Level
The highest water level recorded over the period of record.


IGLD (1955)
International Great Lakes Datum 1955
IGLD (1985)
International Great Lakes Datum 1985
International Great Lakes Datum (1985) [IGLD 1985]

A datum established by the Canada-U.S. Coordinating Committee on Great Lakes Basic Hydraulic and Hydrological Data. It provides a unified datum for use in hydraulic and hydrological studies on both sides of the border along the Great Lakes and St. Lawrence River.

International Hydrographic Organization (IHO)

The International Hydrographic Organization is an intergovernmental consultative and technical organization. It was established in 1921 to support the safety in navigation and the protection of the marine environment.

The object of the organization is to bring about the:

  • coordination of the activities of national hydrographic offices
  • greatest possible uniformity in nautical charts and documents
  • adoption of reliable and efficient methods of carrying out and exploiting hydrographic surveys
  • development of the sciences in the field of hydrography and the techniques employed in descriptive oceanograph



A division of the log line by which the ship’s speed is measured. A nautical unit of speed where 1 knot is:

  • 1 nautical mile per hour
  • 1852 metres or 6076.115,49 international feet per hour

The name is derived from the knots in the log line.


Lagging of tide
The periodic retardation in the time of occurrence of high and low water due to changes in the relative positions of the moon and the sun.
Slight ruffling or roughness on a water surface. Light spray from small waves. Also called leaper.
Lower low water large tide (LLWLT)
The average of the lowest low waters, 1 from each of 19 years of predictions.
Lower low water mean tide (LLWMT)
The average of all the lower low waters from 19 years of predictions.
Lowest normal tide (LNT)
In present usage, it’s synonymous with LLWLT. On older charts it may refer to a variety of low water chart datums.
Lowest Recorded Water Level
The lowest water level recorded over the period of record.
Lunar day

The duration of 1 rotation of the earth on its axis with respect to the moon. Its average length is about 24 hours and 50 minutes of mean solar time.

The duration of 1 rotation of the moon on its axis with respect to the sun is called tidal day.

Lunar tide
That part of the tide due solely to the tide-producing forces of the moon. This is distinguished from that part caused by the forces of the sun.


Any graphic representation of the rise and fall of the tide. Time is generally represented by the absciss and the height of the tide by ordinates.
Mountain Daylight Standard Time
Mean water level (MWL)
The average of all hourly water levels over the available period of record.
Mixed (current)
A type of tidal current characterized by a conspicuous difference in speed and duration between the 2 flood currents or 2 ebb currents occurring each tidal day.
Mixed (tide)

A type of tide in which a diurnal wave produces large inequalities in heights and/or durations of successive high and low waters.

This term applies to the tides intermediate to those predominantly semidiurnal and those predominantly diurnal.

Mountain Standard Time, UTC-7


Newfoundland Daylight Standard Time
Neap tides or tidal currents
The tides of decreased range or tidal currents of decreased speed occurring near the times of the first and last quarters of the moon.
Newfoundland Standard Time, UTC-3 1/2



Water levels that are observed, measured and recorded on site at tidal and non-tidal stations across Canada. These measurements are typically referenced to chart datum.


A shallow water harmonic tide constituent with a speed that is a multiple of the speed of 1 of the basic constituents of the tide-producing force.

See harmonic constituent, and shallow water constituent.


Pacific Daylight Standard Time
That orbital point nearest the earth when the earth is the centre of attraction. The opposite of apogee.
That orbital point nearest the sun, when the sun is the centre of attraction. The opposite of aphelion.
Phases of the Moon

The various appearances of the moon during different parts of the synodical month.

The cycle begins with the new moon or change of the moon at conjunction. The visible part of the waxing moon increases in size during the first half of the cycle, until the full moon appears at opposition. After which, the visible part of the waning moon decreases for the remainder of the cycle.

The first quarter occurs when the waxing moon is at east quadrature. The last quarter occurs when the waning moon is at west quadrature.

The moon is crescent from the last quarter to the new and from the first quarter. It’s gibbous from the first quarter to full and from full to last quarter.

Permanent non-operating
Permanent operating
Pacific Standard Time, UTC-8


Quadrature of moon

An elongation of 90° usually specified as east or west in accordance with the direction of the body from the sun. The moon is at quadrature at first and last quarters.

See phases of the moon. The situation of 2 periodic quantities differing by a quarter of a cycle.



A race is:

  • swiftly flowing water in a narrow channel or river
  • the channel itself which may be artificial (mill-race)
  • a swift rush of water through a narrow channel in tidal waters and caused by the tidal movement of the waters (tide race)
Range of tide
The difference in height between consecutive high and low tides at a place. Also called tidal (or tide) range.
Reduction of tides or tidal currents
A processing of observed tide or tidal current data to obtain mean values for tidal or tidal current constants.
Reference station (ports)

A place where tide or tidal current constants have been determined from observations. This determination is used as a standard for the comparison of simultaneous observations at a subordinate station.

It’s also a place for which independent daily predictions are given in the tide or tidal current tables. From these, corresponding predictions are obtained for other locations by means of differences or factors.

The accuracy of the predictions for reference ports depends on the quantity and quality of the tidal constants used to compute them. These, in turn, are directly related to the length of the period of observations used in the harmonic analysis from which the constants were derived. Whenever the period of record permits, observations extending over at least 1 year are used.

Residual current
The observed current minus the astronomical tidal current or the mean current without periodic components.
Reversing current

A tidal current which flows alternately in approximately opposite directions, with slack water at each reversal. Such currents occur principally in areas where motion is largely restricted to relatively narrow channels.

Also called rectilinear current or rectilinear stream.

Reversing Falls

Time of slack waters for the end of the inward run (flood) and outward run (ebb) at the Reversing Falls on the Saint John River, New Brunswick.

Rip current
A narrow intense current setting seaward through the surf zone. It removes the excess water brought to the zone by the small net mass transport of waves. It's fed by long shore currents. Rip currents usually occur at points, groins, jetties, etc., of irregular beaches, and at regular intervals along straight, uninterrupted beaches.
Rise of tide
The height of high water above datum. Also called tidal rise.
Rotary current
A tidal current that flows continually, with the direction of flow changing through 360° during a tidal cycle. Called rotary stream in British terminology.


Secondary control tide station
A tide station at which continuous observations have been made over a minimum period of time. A secondary tide station is operated during a shorter period of time than a reference station to obtain data for a specific purpose.
Secondary ports

Secondary ports are those which are provided:

  • time and height differences relative to a reference port
  • time differences and rate factors relative to a reference current station

The accuracy of the tidal differences for secondary ports also depends on the quality of the tidal constants used to compute them. In most cases, the period of observations doesn’t extend over 1 month and may be less. Their quality is affected by the amount the tide levels fluctuated from normal during that period on account of meteorological conditions.

Their accuracy is also very dependent on the similarity between the characteristics of the tide at the secondary and reference ports. Tides are unique around the world. Therefore, even when their characteristics are similar, the secondary port predictions made by applying tidal differences can never be considered as accurate as the full predictions made for a reference port. Every effort has been made to compare reference and secondary ports which have similar tidal characteristics.

However, because of the relatively small number of reference ports available, this hasn’t always been possible. Thus, the inaccuracies created are usually less than those caused by fluctuations in the tide levels due to meteorological conditions.


A standing wave oscillation of an enclosed or semi-enclosed water body that continues, pendulum fashion, after the cessation of the originating force. The original force may have been:

  • seismic
  • atmospheric
  • wave induced
Shoreline (coastline)
The line where shore and water meet. Although the terminology of coasts and shores is rather confused, shoreline and coastline are generally used as synonyms.
Slack water (slack or turn)
The interval when the speed of the tidal current is very weak or 0. Usually refers to the period of reversal between ebb and flood currents. Can also be called slack tide.
Sounding datum
Solar tide
That part of the tide due solely to the tide-producing forces of the sun. This is distinguished from that part caused by the forces of the moon.
Solar time

Time based upon the rotation of the earth relative to the sun.

See time:

  • civil
  • mean solar
  • astronomical
  • apparent solar

The solstice is 1 of the 2 points of the ecliptic farthest from the celestial equator. It’s 1 of the 2 points on the celestial sphere occupied by the sun at maximum north or south declination.

That, in the northern hemisphere, is called the summer solstice or first point of Cancer. In the southern hemisphere it’s called the winter solstice or first point of Capricorn.

Also called solsticial point, which is the instant at which the sun reaches 1 of the solstices. This occurs around June 21 (summer solstice) or December 22 (winter solstice) in the northern hemisphere.

Spring tide

The tides of increased range occurring near the times of full moon and new moon.

When those tides occurring near the equinoxes, when the full or new moon and the sun have little or low declination. Also when the spring tides of greater range than the average occur, particularly if the moon is also nearly in perigee.

Stand of tide

Stand of tide is the condition at high tide or low tide when there’s no change in the height of the water.

It may be called high water stand if it occurs at the time of high tide, and low water stand if it occurs at low tide. Also called stand.

Storm surge

A storm surge is a rise above normal water level on the open coast due only to the action of wind stress on the water surface.

Storm surge resulting from a hurricane or other intense storm also includes the rise in level due to atmospheric pressure reduction as well as that due to wind stress. A storm surge is more severe when it occurs in conjunction with a high tide.

Also called:

  • storm tide
  • tidal wave
  • storm wave
In astronomy, either of 2 opposing points in the orbit of a planet or satellite, especially of the moon, at which it’s in conjunction with or in opposition to the sun. Position of the moon when it’s new or full.


Tidal constants

Tidal relations that remain essentially constant for any particular locality. Tidal constants are classed as harmonic and non-harmonic.

The harmonic constants consist of the amplitudes and epochs.

The non-harmonic constants, including those values determined directly from observations, such as tidal ranges and intervals.

Tidal current
The alternating horizontal movement of water associated with the rise and fall of the tide caused by tide-producing forces. Also called tidal stream.
Tidal difference
Difference in time or height of a high or low water at a subordinate station and at a reference station for which predictions are given in the tide tables. The difference, when applied according to sign to the prediction at the reference station, gives the corresponding time or height for the subordinate station.
Tidal Predictions
Estimation of past, present and future water levels. This estimation is based on a calculation method that mainly takes into account astronomical phenomena. Does not include meteorological effects on water levels.
Tidal wave
The wave motion of the tides. In popular usage, any unusually high (and therefore destructive) water level along a shore. The term tidal wave usually refers to either a storm surge or tsunami.
The periodic rise and fall of the surface of oceans, bays, etc., due principally to the gravitational interactions between the moon, sun and earth.
Tide (water level) gauge

A device for measuring the height of tide, such as:

  • a graduated staff in a sheltered area where visual observations can be made
  • an elaborate recording instrument making a continuous graphic record of tide height against time

The elaborate instrument is usually actuated by a float in a pipe communicating with the sea through a small hole which filters out shorter waves.

Tide curve
Any graphic representation of the rise and fall of the tide. Time is generally represented by the abscissa and the height of the tide by ordinates. For normal tides the curve so produced approximates a sine curve. Also called marigram.
Tide reducer
The correction that must be applied to a recorded sounding for the height of tide above or below the datum of reference at the time of sounding.
Tide rip(s)
Small waves formed on the surface of water by the meeting of opposing tidal currents or by a tidal current crossing an irregular bottom. Vertical oscillation, rather than progressive waves, is characteristic of tide rips.
Tide staff
A tide gauge consisting of a vertical graduated pole from which the height of tide at any time can be read directly. Also called tide pole.
Tide tables

Tide tables provide predicted times and heights of the high and low waters associated with the vertical movement of the tide. These tables are necessary for:

  • anchoring
  • establishing the appropriate times for beaching a boat
  • obtaining the depth of water under the keel or over a shoal
Tide-producing force(s)

That part of the gravitational attraction of a celestial body which is effective in producing the tides on earth.

The sun and moon are the principal astronomic bodies that have a tide-producing effect. The force varies approximately as the mass of the attracting body and inversely as the cube of its distance. Also called tide-generating force.

Time zones
  • UTC (0) Coordinated Universal Time
  • NDT (UTC-2.5) Newfoundland Daylight Time
  • NST (UTC-3.5) Newfoundland Standard Time
  • ADT (UTC-3) Atlantic Daylight Time
  • AST (UTC-4) Atlantic Standard Time
  • EDT (UTC-4) Eastern Daylight Time
  • EST (UTC-5) Eastern Standard Time
  • CDT (UTC-5) Central Daylight Time
  • CST (UTC-6) Central Standard Time
  • MDT (UTC-6) Mountain Daylight Time
  • MST (UTC-7) Mountain Standard Time
  • PDT (UTC-7) Pacific Daylight Time
  • PST (UTC-8) Pacific Standard Time
Temporary non-operating
Temporary operating

A long-period sea wave that builds up to great heights over shoal water. It’s produced by a submarine earthquake or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin.

Also called:

  • tsunami
  • tidal wave
  • seismic sea wave


United States Lake Survey 903 adjustment
Universal Time Co-ordinated


Vanishing tide
The phenomenon occurring when a high and low water 'melt' together into a period of several hours with a nearly constant water level. The tide is in the diurnal category but is known as a 'vanishing tide'.
Velocity (of current)
A vector quantity equal to speed in a given direction. Speed and set of the current.


A disturbance which moves through or over the surface of the medium with speed dependent upon the properties of the medium.
Wave height
The vertical distance between crest and trough.