A model of the Earth's surface, or part of the Earth's surface, upon which a coordinate system using angles (i.e. latitude and longitude) can be based.

Datums1 are the province of surveying and the science of geodesy and independent of any one map projection.  For our purpose it is sufficient to state:

• Each datum measures latitude and longitude for any point on the Earth's surface relative to a single point. Sometimes, the point is a master control station for a country's survey network, other times it is the center of the Earth. To be truthful, this reference point is the thing we should call "datum".  But since the reference point gives rise to the entire model, we use "datum" to mean the entire model.
• Each datum optimizes the accuracy of latitude and longitude for a certain portion of the Earth's surface.
• Each datum takes a certain geoid and projects it onto a particular ellipsoid for the purpose of measuring angles.
• The simplest datums possible assume:
• The Earth is a sphere. This is useful for very small-scale (1:14,000,000 or greater) maps of very large areas.
• The Earth is a plane. This is useful for very large-scale (1:20,000 or less) maps of very small areas.
There are hundreds of possible datums. Some important datums are:

• WGS2 84, using the center of the Earth as a reference, measuring latitudes and longitudes with navigation sattelites (e.g. GPS), correcting for gravitational anomalies.
• The 1927 North American Datum (NAD 27), for older maps of the contiguous lower 48 States, referenced to a control station at Meades Ranch in Kansas.
• The 1983 North American Datum (NAD 83), for the entire North American continent. This was originally intended to be based upon a control station in North Dakota, but its reference point is now the center of the Earth. It uses survey data corrected by satellites.
• The International Terrestrial Reference System (ITRS), the most accurate Earth reference system, used for scientific geodesy.

1You could use data as a plural, but that leads to other confusion.

2World Geodetic System

Da"tum (?), n.; pl. Data (#). [L. See 2d Date.]

1.

Something given or admitted; a fact or principle granted; that upon which an inference or an argument is based; -- used chiefly in the plural.

Any writer, therefore, who . . . furnishes us with data sufficient to determine the time in which he wrote. Priestley.

2. pl. Math.

The quantities or relations which are assumed to be given in any problem.

Datum line Surv., the horizontal or base line, from which the heights of points are reckoned or measured, as in the plan of a railway, etc.

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