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A chart datum is the
water level Water level, also known as gauge height or stage, is the elevation of the free surface of a sea, stream, lake or reservoir relative to a specified vertical datum.ISO 772: 1996. Hydrometric determinations – Vocabulary and symbols. See also * ...
surface serving as
origin Origin(s) or The Origin may refer to: Arts, entertainment, and media Comics and manga * Origin (comics), ''Origin'' (comics), a Wolverine comic book mini-series published by Marvel Comics in 2002 * The Origin (Buffy comic), ''The Origin'' (Bu ...
of depths displayed on a
nautical chart A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land (topographic map), natural features of the seabed, details of the coa ...
. A chart datum is generally derived from some
tidal phase Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...
, in which case it is also known as a tidal datum. Common chart datums are ''lowest astronomical tide'' (LAT)Australian Bureau of Meteorolog
National Tide Centre Glossary
(retrieved 30 April 2013)
and ''mean lower low water'' (MLLW). In non-tidal areas, e.g. the
Baltic Sea The Baltic Sea is an arm of the Atlantic Ocean that is enclosed by Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden and the North and Central European Plain. The sea stretches from 53°N to 66°N latitude and from ...
,
mean sea level There are several kinds of mean in mathematics Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. ...
(MSL) is used. A chart datum is a type of
vertical datum In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum, is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built stru ...
and must not be confused with the
horizontal datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other plan ...
for the chart.


Definitions

The following
tidal phase Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...
s are commonly used in the definition of chart datums.


Lowest astronomical tide

''Lowest astronomical tide'' (LAT) is defined as the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. Many national charting agencies, including the
United Kingdom Hydrographic Office The United Kingdom Hydrographic Office (UKHO) is the UK's agency for providing hydrographic and marine geospatial data to mariners and maritime organisations across the world. The UKHO is a trading fund of the Ministry of Defence (MoD) and is ...
and the
Australian Hydrographic Service The Australian Hydrographic Service (formerly known as the Royal Australian Navy Hydrographic Service) is the Australian Commonwealth Government agency responsible for providing hydrographic services that meet Australia's obligations under the SO ...
,Australian Tides Manual SP9 V4.1
Australian Intergovernmental Committee on Surveying and Mapping, Permanent Committee on Tides and Mean Sea Level
use the LAT to define chart datums. One advantage of using LAT for chart datums is that all predicted tidal heights must then be positive (or zero) avoiding possible ambiguity and the need to explicitly state sign. Calculation of the LAT only allows for gravitational effects so lower tides may occur in practice due to meteorological effects, such as high pressure systems.


Mean high water

''Mean high water'' (MHW) is the
average In ordinary language, an average is a single number taken as representative of a list of numbers, usually the sum of the numbers divided by how many numbers are in the list (the arithmetic mean). For example, the average of the numbers 2, 3, 4, 7, ...
of all the daily tidal high water levels observed over a period of several years. In the United States this period spans 19 years and is referred to as the
National Tidal Datum Epoch A chart datum is the water level surface serving as origin of depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are ''lowest ast ...
. In
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, the definition of the MHW is Ladder '...the line of the medium high tide between the highest tide of each lunar month (the springs) and the lowest each lunar month (the Neaps) averaged over the year.'


Mean water


Mean lower low water

''Mean lower low water'' (MLLW) is the average height of the lowest tide recorded at a tide station each day during a 19-year recording period, known as the ''National Tidal Datum Epoch'' as used by the United States'
National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (abbreviated as NOAA ) is an United States scientific and regulatory agency within the United States Department of Commerce that forecasts weather, monitors oceanic and atmospheric conditio ...
. MLLW is only a mean, so some tidal levels may be negative relative to MLLW; see also #Mean low water springs. The 19-year recording period is the nearest full year count to the 18.6-year cycle of the
lunar node A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersects the ecliptic. The ''ascending'' (or ''north'') node is where the Moon moves into the northern ecliptic hemisphere, w ...
regression, which has an effect on tides.


Lower low water large tide

This is an average of lowest low waters taken over a fixed period of tidal predictions, as opposed to actual observations. This is the datum used for coastal charts published by the
Canadian Hydrographic Service ''Retired Canadian Hydrographic Service logo or crest'' The Canadian Hydrographic Service (CHS) is part of the federal department of Fisheries and Oceans Canada and is Canada's authoritative hydrographic office. The CHS represents Canada in t ...
, with the average taken from 19 years of tidal predictions.


Mean higher high water

Similarly, the ''mean higher high water'' (MHHW) is the average height of the highest tide recorded at a tide station each day during the recording period. It is used, among other things as a datum from which to measure the navigational clearance, or
air draft Air draft (or air draught) is the distance from the surface of the water to the highest point on a vessel. This is similar to the " deep draft" of a vessel which is measured from the surface of the water to the deepest part of the hull below th ...
, under bridges.


Mean water spring

Spring tide Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables ca ...
s are those when the
moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...
is in a direct alignment with the sun (thus new or full) and in many extra-tropics places when its
declination In astronomy, declination (abbreviated dec; symbol ''δ'') is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the ...
is 23.5°, its maximum. In equatorial, tropical seas, such as the
Banda Sea The Banda Sea ( id, Laut Banda, pt, Mar de Banda, tet, Tasi Banda) is one of four seas that surround the Maluku Islands of Indonesia, connected to the Pacific Ocean, but surrounded by hundreds of islands, including Timor, as well as the Halma ...
such tides (bulges) occur when there is such an alignment and the declination of the moon is more towards its 0° average, thus more overhead or antiposed.


Mean low water springs

''Mean low water springs'' (MLWS) is the average of the water levels of each pair of successive low waters during that period of about 24 hours in each semi-lunation (approximately every 14 days), when the
tidal range Tidal range is the difference in height between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun and the rotation of Earth. Tidal range depends on time and location. ...
is greatest (spring range).Definitions of various tide related terms
Proudman Oceanographic Laboratory The former Proudman Oceanographic Laboratory (POL) is based in Brownlow Street, Liverpool, England. In April 2010, POL merged with the National Oceanography Centre, Southampton (NOCS) to form the National Oceanography Centre. The Liverpool labora ...
.


Mean high water springs

''Mean high water springs'' (MHWS) is the averaged highest level that spring tides reach over many years (often the last 19 years). Within this, to ensure anomalous levels are tempered, at least two successive
high water Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon (and to a much lesser extent, the Sun) and are also caused by the Earth and Moon orbiting one another. Tide tables can ...
s during the highest-tide 24 hours are taken. Such a local level is generally close to the "
high water mark A high water mark is a point that represents the maximum rise of a body of water over land. Such a mark is often the result of a flood, but high water marks may reflect an all-time high, an annual high (highest level to which water rose that ...
" where debris accumulates on a tidal shore on about two days six months apart (and nearby days) annually. The levels are local as some places are nearer to or form places of almost no tides in and around each ocean (
amphidromic point An amphidromic point, also called a tidal node, is a geographical location which has zero tidal amplitude for one harmonic constituent of the tide. The tidal range (the peak-to-peak amplitude, or the height difference between high tide and low ...
s).


Usage


Charts and tables

Charted depths and
drying height On a nautical chart, the drying height is the vertical distance of the seabed that is exposed by the tide, above the sea water level at the lowest astronomical tide. On admiralty chart Admiralty charts are nautical charts issued by the United K ...
s on
nautical chart A nautical chart is a graphic representation of a sea area and adjacent coastal regions. Depending on the scale of the chart, it may show depths of water and heights of land (topographic map), natural features of the seabed, details of the coa ...
s are given relative to chart datum. Some height values on charts, such as vertical clearances under bridges or overhead wires, may be referenced to a different vertical datum, such as
mean high water spring A chart datum is the water level surface serving as origin of depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are ''lowest astr ...
s or highest astronomical tide (HAT) (for "HAT" see
tidal range Tidal range is the difference in height between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun and the rotation of Earth. Tidal range depends on time and location. ...
).
Tide table Tide tables, sometimes called tide charts, are used for tidal prediction and show the daily times and levels of high and low tides, usually for a particular location. Tide heights at intermediate times (between high and low water) can be approxim ...
s give the height of the tide above a chart datum making it feasible to calculate the depth of water at a given point ''and'' at a given time by adding the charted depth to the height of the tide. One may calculate whether an area that dries is under water by subtracting the drying height from the
iven Iven is a municipality in the Vorpommern-Greifswald district, in Mecklenburg-Vorpommern, Germany. Notable residents *Georg Detlev von Flemming Georg Detlev von Flemming (Polish: Jerzy Detloff Fleming) (3 March 1699 – 10 December 1771) was a ...
height calculated from the tide table. Using charts and tables not based on the same
geodetic datum A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other plan ...
can result in incorrect calculation of water depths.


Satellite navigation

In recent years national hydrographic agencies have spearheaded developments to establish chart datum with respect to the
Geodetic Reference System 1980 The Geodetic Reference System 1980 (GRS 80) is a geodetic reference system consisting of a global reference ellipsoid and a normal gravity model. Background Geodesy is the scientific discipline that deals with the measurement and representation ...
(GRS 80) reference ellipsoid, thus enabling direct compatibility with
satellite navigation A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location (longitude, latitude, and altitude/elevation) to high pr ...
(GNSS) positioning. Examples of this include
Vertical Offshore Reference Frames Vertical Offshore Reference Frames (VORF) is a set of high resolution surfaces which together define a vertical datum for hydrographic surveying and charting in the United Kingdom and Ireland. The following surfaces are included: * Chart Datum (C ...
(VORF) for the
United Kingdom Hydrographic Office The United Kingdom Hydrographic Office (UKHO) is the UK's agency for providing hydrographic and marine geospatial data to mariners and maritime organisations across the world. The UKHO is a trading fund of the Ministry of Defence (MoD) and is ...
(UKHO) and Bathyelli for
Naval Hydrographic and Oceanographic Service The Naval Hydrographic and Oceanographic Service (french: Service hydrographique et océanographique de la Marine or SHOM) is a French public establishment of an administrative nature (french: établissement public à caractère administratif) admi ...
(SHOM).


References

{{reflist Cartography Tide tables Vertical datums Nautical charts Zero-level elevation points