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
2World Geodetic System