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A tide gauge is a device for measuring the change in sea level relative to a vertical datum. [ 2 ] [ 3 ] It is also known as a mareograph , [ 4 ] marigraph , [ 5 ] and sea-level recorder . [ 6 ] When applied to freshwater continental water bodies , the instrument may also be called a limnimeter .
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 main product of the VORF project was the gridded vertical correction files which deliver the capability to transfer heights and depths from one vertical reference system to another, "allowing the direct use of depth data from surveys which is referred to a WGS84 compatible datum rather than Chart Datum and thus enabling Hydrographic surveyors to survey without the need to measure tides".
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.
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).
At these points in the lunar cycle, the tide's range is at its minimum; this is called the neap tide, or neaps. "Neap" is an Anglo-Saxon word meaning "without the power", as in forðganges nip (forth-going without-the-power). [9] Neap tides are sometimes referred to as quadrature tides. [8]
For instance the chart for 3 hours after high water is valid from HW+2½ to HW+3½, not from HW+3 to HW+4. [2] Tidal atlases may provide additional information for areas such as estuaries where it important to calculate tides away from the ports. Such information may include co-tidal range information [3] and time differences. [4]
If a tide table gives the information that tomorrow's low water would be at noon and that the water level at this time would be two metres above chart datum, and that at the following high tide the water level would be 14 metres, then the height of water at 3:00 p.m. can be calculated as follows: