Definition of "datum"

datum

/ˈdeɪtəm/

noun
plural (senses 1–3) data or (senses 1 and 4–5) datums
1. (dated) Singular of data; a single recorded observation.
2. (philosophy) A fact known from direct observation.
3. (philosophy) A premise from which conclusions are drawn.
4. (cartography, surveying, engineering) A fixed reference point or set of reference points which precisely define a system of measurement or a coordinate system.
  Quotations
  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.).
  2000, Nuno Sergio Marques Antunes, “The Importance of the Tidal Datum in the Definition of Maritime Limits and Boundaries”, in Maritime Briefing, volume 2, number 7, International Boundaries Research Unit, University of Durham, page 5
  Datums are another important map aspect related to projection. A datum provides a base reference for measuring locations on Earth's surface.
  2007, Roger F Tomlinson, Thinking about GIS: geographic information system planning for managers
  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.
  2012, Yong-Qi Chen, Yuk-Cheung Lee, chapter 2.3, in Geographical Data Acquisition
5. (nautical) A floating reference point, or SLDMB, used to evaluate surface currents in a body of water. Often employed by coastal search and rescue.
verb
third-person singular simple present datums, present participle datuming or datumming, simple past and past participle datumed or datummed
1. To provide missing data points by using a mathematical model to extrapolate values that are outside the range of a measuring device.
  Quotations
  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.
  1982, Paul M. Tucker, Pitfalls Revisited - Issue 3, page 6
  On the left the stacking velocity functions are datumed to sea level and show great disparity.
  1998, Stuart Fagin, Model-based Depth Imaging, page 164
  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).
  2014, Hua-Wei Zhou -, Practical Seismic Data Analysis, page 62