A chart datum is the

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

surface serving as origin (or coordinate surface) of depths displayed on a

nautical chart A nautical chart or hydrographic chart is a graphic representation of a sea region or water body and adjacent coasts or river bank, banks. Depending on the scale (map), scale of the chart, it may show depths of water (bathymetry) and heights of ...

and for reporting and predicting tide heights. 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 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 the countries of Denmark, Estonia, Finland, Germany, Latvia, Lithuania, Poland, Russia, Sweden, and the North European Plain, North and Central European Plain regions. It is the ...

mean sea level A mean is a quantity representing the "center" of a collection of numbers and is intermediate to the extreme values of the set of numbers. There are several kinds of means (or "measures of central tendency") in mathematics, especially in statist ...

(MSL) is used. A chart datum is a type of vertical datum and must not be confused with the horizontal datum for the chart. However, it is necessarily an equigeopotential (a water "level surface"): the chart datum is tilted across smaller to larger

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, by Earth's rotation and by centrifugal force caused by Earth's prog ...

regions; in rivers, it is a sloping and undulating surface following the low stage.

Definitions

The following tidal phases are commonly used in the definition of chart datums.

Lowest and highest 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 and the Australian Hydrographic Service,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. The ''highest astronomical tide'' (HAT) can be defined similarly.

Mean high water

''Mean high water'' (MHW) is the

average In colloquial, ordinary language, an average is a single number or value that best represents a set of data. The type of average taken as most typically representative of a list of numbers is the arithmetic mean the sum of the numbers divided by ...

of all the daily tidal high water levels observed over a period of several years. It is not the same as the normal tidal limit. In the United States this period spans 19 years and is referred to as the ''National Tidal Datum Epoch'' as used by the United States'

National Oceanic and Atmospheric Administration The National Oceanic and Atmospheric Administration (NOAA ) is an American scientific and regulatory agency charged with Weather forecasting, forecasting weather, monitoring oceanic and atmospheric conditions, Hydrography, charting the seas, ...

. In

Australia Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...

, the definition of the MHW is '...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. 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 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, with the average taken from the lowest tides, one from each 19 year period 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, 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 ...

s are those when the

moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

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. The declination angle is measured north (positive) or ...

is 23.5°, its maximum. In equatorial, tropical seas, such as the

Banda Sea The Banda Sea (, , ) 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 Halmahera Sea, Halmahera and Ceram Seas. It is about ...

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

is greatest (spring range).Definitions of various tide related terms
Proudman Oceanographic Laboratory.

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 High Water or Highwater may refer to: * High water, the state of tide when the water rises to its highest level. Film and television * Highwater (film), ''Highwater'' (film), a 2008 documentary * ''Step Up: High Water'', a web television series * ...

s during the highest-tide 24 hours are taken. Such a local level is generally close to the " high water mark" 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 points).

Usage

Charts and tables

Charted depths and drying heights on

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 springs or highest astronomical tide (HAT) (for "HAT" see

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 approxi ...

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 ivenheight 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, or terrestrial reference frame) is a global datum reference or reference frame for unambiguously representing the positi ...

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 (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 geopositioning. A satellite navigation system with global coverage is termed global navigation satellite system (GNSS). , four global systems are ope ...

(GNSS) positioning. Examples of this include Vertical Offshore Reference Frames (VORF) for the United Kingdom Hydrographic Office (UKHO) and Bathyelli for Naval Hydrographic and Oceanographic Service (SHOM).

References

{{reflist Cartography Tide tables Vertical datums Nautical charts