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. A nautical chart is a graphic representation of a maritime area and adjacent Coastal regions The chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood This article describes a concept from Surveying and Geodesy. For other meanings see Datum (disambiguation. 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 United Kingdom Hydrographic Office (or UKHO) is an organisation within the UK government responsible for providing Navigational and other hydrographic The Admiralty was formerly the authority in the United Kingdom responsible for the command of the Royal Navy. Characteristics A tide is a repeated cycle of sea level changes in the following stages Over several hours the water rises or advances up a beach in the flood 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). Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface

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. The National Oceanic and Atmospheric Administration ( NOAA) is a scientific agency within the United States Department of Commerce focused on the conditions of the 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. 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 A nautical chart is a graphic representation of a maritime area and adjacent Coastal regions 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. A tide table, sometimes called a tide chart, is used for tidal prediction and shows the daily times and height of High water and Low water 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.