Editing Datum reference (section)

{{short description|A point of reference against which some others are calculated}}
{{Refimprove|date=November 2015}}
[[File:Geometric Tolerancing Example.PNG|thumb|right|Technical drawing with references and geometric specifications]]

A '''datum reference''' or just '''datum''' (plural: '''datums'''<ref name="plural" group="Note">The plural of this [[word sense|sense]] of the word ''[[wikt:datum#Noun|datum]]'' is ''datums'' by convention, in contrast with the other senses of the word in which ''[[data]]'' usually serves as both the plural form and the [[mass noun]] counterpart.) </ref>) is some [[Geometry|geometrically]] important part of an object—such as a [[point (geometry)|point]], [[line (geometry)|line]], [[plane (geometry)|plane]], hole, set of holes, or pair of [[surface (mathematics)|surface]]s—that serves as a reference in defining the [[geometry]] of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the [[real versus nominal value|nominal]] value, which may be an ideal, standard, average, or desired value.

For example, on a car's wheel, the [[lug nut]] holes define a [[wikt:bolt circle#Noun|bolt circle]] that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be [[concentric objects|concentric]] to within close limits (or else the wheel will not roll smoothly).

The concept of datums is used in many fields, including [[carpentry]], [[metalworking]], [[needlework]], [[geometric dimensioning and tolerancing]] (GD&T), [[aviation]], [[surveying]], [[geodesy]] ([[geodetic datum]]s), and others.