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 Admira ...

s and others 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 u ...

in a

rhombus In plane Euclidean geometry, a rhombus (: 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. The rhom ...

, printed in purple ink. On any particular chart each tidal diamond will have a unique letter starting from "A" and continuing alphabetically. Either somewhere on the chart (generally on land) or else on a separate sheet, 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 or Highwater may refer to: * High water, the state of tide when the water rises to its highest level. Film and television * Highwater (film), ''Highwater'' (film), a 2008 documentary * ''Step Up: High Water'', a web television series * ...

, high water itself and the 6 hours after high water. It is important to realise that the tidal hour lasts from half an hour before the nominal time to half an hour after. For instance the row for 3 hours after high water is valid from HW+2½ to HW+3½, not from HW+3 to HW+4. 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 c ...

. The times on the table are related to the high water of the standard port displayed on the table. To calculate the rate at an intermediate tide between neap and spring, interpolation is required. Traditionally this has been done using a "calculation of rates" chart. 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 arrow ...

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

Footnotes

References

Citations

* * *{{Cite book, edition = Edition 6, publisher = United Kingdom Hydrographic Office, isbn = 978-0-7077-4174-1, author = United Kingdom Hydrographic Office, title = Symbols and abbreviations used on admiralty paper charts, location = Taunton, Somerset, date = 2016 Navigation Tides