Tidal diamonds are symbols on British

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 Admi ...

s that indicate the direction and speed of tidal streams. The symbols consist of a letter of the

ISO basic Latin alphabet The ISO basic Latin alphabet is an international standard (beginning with ISO/IEC 646) for a Latin-script alphabet that consists of two sets (uppercase and lowercase) of 26 letters, codified in various national and international standards and ...

in a

rhombus In plane Euclidean geometry, a rhombus (plural rhombi or rhombuses) is a 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. Th ...

, 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 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 c ...

, high water itself and the 6 hours after high water.EasyTide - on-line tidal predictions from the UKHO
/ref> The columns show the bearing of the tidal stream and its speed, in

knot A knot is an intentional complication in Rope, cordage which may be practical or decorative, or both. Practical knots are classified by function, including List of hitch knots, hitches, List of bend knots, bends, List of loop knots, loop knots, ...

s, at both

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 ...

and

neap 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 can ...

. The times on the table are related to the high water of the

standard port A Standard port is a port whose tidal predictions are directly given in the Tide tables. Tide predictions for standard ports are based on continuous observation of tide over a period of at least one year. These predictions are given in feet or mete ...

displayed on the table. An alternative to a tidal diamond is a

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 ...

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

References

{{Reflist Navigation Tides