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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 ...
Consequently, all navigators use tide tables either in a booklet, computer or digital tide clock. Analog tide clocks are most accurate for use on the Atlantic coasts of America and Europe. This is because along the Atlantic coastline the moon controls the tides predictably, ebbing and flowing on a regular (12- to 13-hour) schedule.
Each day, tides force large amounts of seawater through the narrows—760,000,000 m 3 (200 × 10 ^ 9 US gal) of water on a 3 m (9.8 ft) tide. The difference in water levels on either side of the rapids can exceed 2 m (6 ft 7 in) in height. Current speeds can exceed 16 knots (30 km/h; 18 mph), [2] up to 17.68 kn (32.74 km/h; 20.35 mph). [3]
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).
Tidal range is the difference in height between high tide and low tide. Tides are the rise and fall of sea levels caused by gravitational forces exerted by the Moon and Sun, by Earth's rotation and by centrifugal force caused by Earth's progression around the Earth-Moon barycenter. Tidal range depends on time and location.
Tide flow information is most commonly seen on nautical charts, presented as a table of flow speeds and bearings at hourly intervals, with separate tables for spring and neap tides. The timing is relative to high water at some harbour where the tidal behaviour is similar in pattern, though it may be far away.
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On nautical charts, the top of the chart is always true north, rather than magnetic north, towards which a compass points. Most charts include a compass rose depicting the variation between magnetic and true north. However, the use of the Mercator projection has drawbacks. This projection shows the lines of longitude as parallel.