In a strict sense, a tidal datum can be understood as the reference plane (or surface) to which the height of a predicted tide is referred.[…]Sounding and chart datums are low water datums, that is, they refer to the level of the water surface at low tide. Nonetheless, there are also datums based on high water levels.[…]These two different datums may be included in the broader category of vertical datums, which comprises any plane or surface used as a reference to measure vertical distances (such as depths, drying features, heights on shore, etc.).

Datums are another important map aspect related to projection. A datum provides a base reference for measuring locations on Earth's surface.

For horizontal measurements [on the Earth], we fix a mathematical body of Earth in space using a Cartesian coordinate system. After that, a separate coordinate system is created over the surface of this body to generate horizontal coordinates. A mathematical earth body fixed in space makes up the horizontal datum.

Removing the effects of any period of deformation by datuming or flattening selective reflection horizons should restore the structure prior to the datumed horizon, or the amount of deformation above the datumed horizon.

On the left the stacking velocity functions are datumed to sea level and show great disparity.

On the other hand, if we have a sufficiently accurate near-surface velocity model, we may apply wavefield datuming to convert the raw data into new data as if they were recorded along a datum below the near surface (Box 2.3).

Datum

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