Datums and Reference Frames


A geodetic datum or reference frame is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps.

NGS defines the official geodetic datums for all federal mapping activities in the U.S. and its territories (see Table 1) as part of the National Spatial Reference System (NSRS). However, NGS will be defining “new datums” to replace these datums.

TABLE 1: Current Datums of the United States and Territories

Current official datum

Description

This geometric datum allows measurement of positions relative to an ellipsoid model of the Earth (latitude, longitude and ellipsoid height).

It replaced previous “horizontal datums” that only provided latitude and longitude information.
This vertical datum allows measurement of land elevations and water depths

International Reference Frames

A Terrestrial Reference frame provides a set of coordinates of some points located on the Earth's surface. NGS contributes to International bodies that define globally consistent reference frames, and to some other frames that are used to define a national coordinate system or datum (see Table 2).

TABLE 2: International Reference Frames

Name

Description

International Terrestrial Reference Frame (ITRF)
The current global standard reference frame. NGS New datums will align with the current ITRF.
World Geodetic System 1984 (WGS 84)
Defined by the U.S. Department of Defense. Commonly used within civilian GPS software.
Geodetic Reference System 1980 (GRS 80)
Geodetic reference system consisting of a global reference ellipsoid and a gravity field model. Basis for NAD 83 and related datums.