Spatial - Ed.com

A datum is a point of reference from which measurements are made.

It is where the coordinate grid is tied to the earth’s surface. Datums are recalculated and updated as technology allows for more precise measurements. These updates are referred to as realizations, flavors or many similar terms. The most commonly encountered datums are -

World Geodetic System 1984 (WGS-84)

GPS satellites broadcast signals in WGS-84 and most GPS receivers, including Trimble mobile devices, calculate data in WGS-84. The latest realization of WGS-84 is WGS84(G1150).

International Terrestrial Reference Frame (ITRF00)

The latest realization is ITRF00 (aka ITRF2000). WGS84(G1150) and ITRF00 are within millimeters of each other and can be considered equivalent.

North American Datum 1983 (NAD83)

Many maps in the United States are based on this datum. The realization most currently used is NAD83(CORS96) with specific updates for Alaska and the Pacific Islands. The first realizations of WGS-84, WGS-84(1986) and NAD83, NAD83(1986) are equivalent. Through multiple updates of each datum, the difference between the latest realizations, NAD83(CORS96) and WGS-84(G1150), is now many meters in some areas.

Epochs of NAD83 (CORS96)

An epoch is a temporal term that refers to the state of a datum that is fixed to a certain date.

Continental US

The most commonly used epoch of NAD83(CORS96) is NAD83(2002.00) in the Continental US. This epoch of NAD83(CORS96) is referenced to January 1, 2002.

Alaska

The most commonly used epoch of NAD83(CORS96) is NAD83(2002) in Alaska. This epoch of NAD83(CORS96) was adjusted after the 2002 Denali quake.

Pacific Islands

The most commonly used epochs of NAD83(CORS96) CORS in the Pacific are NAD83(PACP00) for the Pacific plate and NAD83(MARP00) for the Mariana plate. These contain parameters to adjust for the tectonic movement of these plates.