Chart Datum

Information about Chart Datum

US civil and maritime uses of tidal data

The chart datum is the level of water that charted depths displayed on nautical charts are measured from. The chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water.

Lowest Astronomical Tide

Many national charting agencies, including the United Kingdom Hydrographic Office and other hydrographic services, such as those of Canada and Australia, that originated with the British Admiralty use the Lowest astronomical tide (LAT), the height of the water at the lowest possible theoretical tide, as chart datum. The advantage of using LAT is that all 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 other factors (e.g. meteorological effects such as high pressure systems).

Mean Lower Low Water

The United States' National Oceanic and Atmospheric Administration uses mean lower low water (MLLW), which is the average of the lowest tide recorded at a tide station each day during the recording period. MLLW is generally located above LAT and therefore some tidal states may have negative heights. The advantage of using Mean Lower Low Water is avoiding the costs and confusion that moving to LAT would involve.

Charts and Tables

Charted depths and drying heights on nautical charts 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.

Tide tables give the height of the tide above chart datum. This makes it possible to calculate the depth of water at a given point and a given time by adding the charted depth to the height of the tide. To calculate whether an area that dries is under water, subtract the drying height from the height of the tide.

Using charts and tables based on a different datum will result in incorrect calculation of water depths and must be avoided. Check that they are compatible before use.

CAUTION: It is dangerous to attempt maritime navigation without proper training.

nautical chart is a graphic representation of a maritime 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 coastline, navigational hazards,
..... Click the link for more information.

Tides are the cyclic rising and falling of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. More generally, tidal phenomena can occur in any object that is subjected to a gravitational field that varies in time and space, such as the
..... Click the link for more information.

This article describes a concept from surveying and geodesy. Datum is also the last known position of a submarine in ASW, the singular form of data, and a city in ancient Macedonia, probably on the site of the modern Kavala.

..... Click the link for more information.

The United Kingdom Hydrographic Office (or UKHO) is an organisation within the UK government responsible for providing navigational and other hydrographic information for national, civil and defence requirements.
..... Click the link for more information.

Naval Service

Components
Royal Navy

Surface Fleet
Fleet Air Arm
Submarine Service
Royal Navy Regulating Branch
Royal Naval Reserve
Queen Alexandra's Royal Naval Nursing Service

Royal Marines

(includes Royal Marines Reserve)

..... Click the link for more information.

Pressure (symbol: p) is the force per unit area applied on a surface in a direction perpendicular to that surface.

Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.
..... Click the link for more information.

National Oceanic and Atmospheric Administration (NOAA) is a scientific agency of the United States Department of Commerce focused on the conditions of the oceans and the atmosphere.
..... Click the link for more information.

On a nautical chart, the drying height is the vertical distance of the seabed that is exposed by the tide, above the level of the sea at the lowest astronomical tide.

On admiralty charts a drying height is distinguished from a depth by being underlined.
..... Click the link for more information.

A tide table is used for tidal prediction and shows the daily times and height of high water and low water for a particular location. Tide heights at intermediate times (between high and low water) can be approximately calculated using the rule of twelfths or more
..... Click the link for more information.

This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.

Herod_Archelaus