Tidal diamond

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A tidal diamond table Nautical chart tidal diamond.PNG
A tidal diamond table

Tidal diamonds are symbols on British admiralty charts that indicate the direction and speed of tidal streams.

Admiralty chart

Admiralty charts are nautical charts issued by the United Kingdom Hydrographic Office (UKHO) and subject to Crown Copyright. Over 3,500 Standard Nautical Charts (SNCs) and 14,000 Electronic Navigational Charts (ENCs) are available with the Admiralty portfolio offering the widest official coverage of international shipping routes and ports, in varying detail.

Tide The periodic change of sea levels caused by the gravitational and inertial effects of the Moon, the Sun and the rotation of the Earth

Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of the Earth.

The symbols consist of a letter of the ISO basic Latin alphabet in a rhombus, printed in purple ink. On any particular chart each tidal diamond will have a unique letter starting from "A" and continuing alphabetically.

The ISO basic Latin alphabet is a Latin-script alphabet and consists of two sets of 26 letters, codified in various national and international standards and used widely in international communication. They are the same letters that comprise the English alphabet.

Rhombus quadrilateral in which all sides have the same length

In plane Euclidean geometry, a rhombus is a simple (non-self-intersecting) quadrilateral whose four sides all have the same length. Another name is equilateral quadrilateral, since equilateral means that all of its sides are equal in length. The rhombus is often called a diamond, after the diamonds suit in playing cards which resembles the projection of an octahedral diamond, or a lozenge, though the former sometimes refers specifically to a rhombus with a 60° angle, and the latter sometimes refers specifically to a rhombus with a 45° angle.

Somewhere on the chart, generally on land, will be a Tidal Diamond table. This contains a grid of thirteen rows and three columns for each Diamond. The rows are the hours of the tidal cycle showing the 6 hours before high water, high water itself and the 6 hours after high water. [1] The columns show the bearing of the tidal stream and its speed, in knots, at both spring tide and neap tide. The times on the table are related to the high water of the standard port displayed on the table.

Bearing (navigation) term used (for example) in navigation

In navigation, bearing is the horizontal angle between the direction of an object and another object, or between it and that of true north.

Knot (unit) unit of speed

The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. The ISO standard symbol for the knot is kn. The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE); kt is also common, especially in aviation where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

A Standard port is a port whose tidal predictions are directly given in the Tide tables.

An alternative to a tidal diamond is a tidal atlas which are often more accurate and easier to use (if available).

A tidal atlas or a tidal stream atlas is used to predict the direction and speed of tidal currents.

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