Tidal diamond
Encyclopedia
Tidal diamonds are symbols on British admiralty chart
Admiralty chart
Admiralty charts are nautical charts issued by the United Kingdom Hydrographic Office and subject to Crown Copyright. Over 3,000 charts are available and cover virtually the entire world in various levels of detail depending on the density of traffic and hazards...

s that indicate the direction and speed of tidal streams
Tide
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 Roman alphabet in a rhombus
Rhombus
In Euclidean geometry, a rhombus or rhomb is a convex quadrilateral whose four sides all have the same length. The rhombus is often called a diamond, after the diamonds suit in playing cards, or a lozenge, though the latter sometimes refers specifically to a rhombus with a 45° angle.Every...

, printed in purple ink. On any particular chart each tidal diamond will have a unique letter starting from "A" and continuing alphabetically.

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
High Water is an album by hip hop artist El-P, released on March 9, 2004 through Thirsty Ear Recordings.Made in conjunction with jazz pianist Matthew Shipp and the group for which he is artistic director, The Blue Series Continuum, the album is a striking departure from El-P's usual style, almost...

, high water itself and the 6 hours after high water. The columns show the bearing
Bearing (navigation)
In marine navigation, a bearing is the direction one object is from another object, usually, the direction of an object from one's own vessel. In aircraft navigation, a bearing is the actual compass direction of the forward course of the aircraft...

 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.

An alternative to a tidal diamond is a tidal atlas
Tidal atlas
A tidal atlas or a tidal stream atlas is used to predict the direction and speed of tidal currents.A tidal atlas usually consists of a set of 12 or 13 diagrams, one for each hour of the tidal cycle, for a coastal region. Each diagram uses arrows to indicate the direction of the flow at that time...

which are often more accurate and easier to use (if available).
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