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Tide tables, sometimes called tide charts, are used for tidal prediction and show the daily times and levels of high and low tides, usually for a particular location. [1] Tide heights at intermediate times (between high and low water) can be approximated by using the rule of twelfths or more accurately calculated by using a published tidal ...
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.
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart and for reporting and predicting tide heights. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. [1] Common chart datums are lowest astronomical tide (LAT) [1] and mean lower low water (MLLW ...
The largest annual tidal range can be expected around the time of the equinox if it coincides with a spring tide. Spring tides occur at the second and fourth (last) quarters of the lunar phases . By contrast, during neap tides , when the Moon and Sun's gravitational force vectors act in quadrature (making a right angle to the Earth's orbit ...
An arrow on the tidal chart indicates the direction and the average flow speed (usually in knots) for spring and neap tides. If a tide chart is not available, most nautical charts have "tidal diamonds" which relate specific points on the chart to a table giving tidal flow direction and speed.
In many parts of the world the tides approximate to a semi-diurnal sine curve, that is there are two high- and two low- tides per day. As an estimate then each period equates to 1 hour, with the tide rising by 1, 2, 3, 3, 2, finally 1 twelfths of its total range in each hour, from low tide to high tide in about 6 hours, then the tide is ...
A Standard port is a port whose tidal predictions are directly given in the Tide tables. [1] 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 meters, with respect to the chart datum for average meteorological conditions. [2]
Depending on the scale of the chart, it may show depths of water and heights of land , natural features of the seabed, details of the coastline, navigational hazards, locations of natural and human-made aids to navigation, information on tides and currents, local details of the Earth's magnetic field, and human-made structures such as harbours ...