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D
- Datum / Élément de référence (Datum)
- Any numerical or geometrical quantity or set of such quantities which may serve as a reference or base for other quantities. In geodesy a datum is defined by a set of parameters specifying the reference surface or the reference coordinate system. Therefore two types of datums are required: a horizontal datum, which forms the basis for computations of horizontal control surveys in which the curvature of the Earth is considered, and a vertical datum, to which elevations are referred.
- Differences / Différences (Les)
- Differences are the adjustments which are applied to the predictions at a reference port or reference current station to obtain predictions at a secondary port or secondary current station.
- Diurnal inequality / Inégalité diurne
- The difference in height of the two high waters, or of the two low waters of each tidal day. Also, the difference in speed between the two flood tidal currents, or the two ebb currents of each tidal day. Diurnal inequality is sometimes called declinational inequality.
- Drift (current) / Courant de dérive
- A wide, slow-moving ocean current principally caused by winds.
- Duration of flood and duration of ebb / Durée du flot et durée du jusant
- Duration of flood is the interval of time in which a tidal current is flooding, and the duration of ebb is the interval in which it is ebbing; these intervals being reckoned from the middle of the intervening slack waters or minimum currents. Together they cover, on an average, a period of 12.42 hours for a semidiurnal tidal current or a period of 24.84 hours for a diurnal current. In a normal semidiurnal tidal current, the duration of flood and duration of ebb will each be approximately equal to 6.21 hours, but the times may be modified greatly by the presence of a non-tidal flow. In a river the duration of ebb is usually longer than the duration of flood because of the fresh water discharge, especially during the spring months when snow and ice melt are the predominant influences.
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| Reviewed:
2003-02-27
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