Geodetic datum

Lore & Background

The spherical nature of Earth was known by ancient Greeks, who developed latitude and longitude and early astronomical methods for measuring them. These methods were preserved and advanced by Muslim and Indian astronomers, sufficient for global exploration in the 15th and 16th centuries. During the Age of Enlightenment, scientific advances revealed errors in these measurements, prompting a demand for greater precision. Isaac Newton theorized that Earth’s rotation should make it oblate, while early surveys by Jacques Cassini suggested a prolate shape.

French geodesic missions to Lapland and Peru confirmed Newton’s hypothesis and also discovered variations in gravity, which later contributed to the geoid model. Concurrently, trigonometric surveys began accurately measuring distances over great extents, starting with Cassini’s surveys and the Anglo-French Survey. By the end of the 18th century, survey control networks covered France and the United Kingdom. More ambitious projects, such as the Struve Geodetic Arc across Eastern Europe and the Great Trigonometrical Survey of India, produced more accurate ellipsoid estimates.

The first triangulation across the United States was completed in 1899. This work led to the North American Datum of 1927 and the Vertical Datum of 1929, the first standard datums for public use. Later, improved measurements, including early satellite data, enabled more accurate regional datums such as NAD83, ETRS89, and GDA94, and global datums like WGS84 for satellite navigation. Local datums can represent a specific area more accurately than a global one, but the global WGS84 has become widely adopted due to its benefits.

Reader's Guide

Geodetic datums are foundational to any technology or technique based on spatial location. Before GPS, there was no precise way to measure positions far from reference points used in local datums, such as the Prime Meridian at Greenwich Observatory, the Equator, or the nearest coast for sea level. Astronomical and chronological methods had limited precision over long distances. The rise of GPS and its WGS 84 datum supplanted most earlier datums in many applications, as WGS 84 is intended for global use.

However, because Earth is an imperfect ellipsoid, local datums can give more accurate representation of specific areas—for example, OSGB36 approximates the geoid over the British Isles better than WGS 84. Despite this, the benefits of a global system have led to widespread adoption of WGS 84. A standard datum specification includes a model for Earth's shape (reference ellipsoid or geoid), an origin tied to a known location, and multiple control points precisely measured from that origin. Coordinates referred to different datums have undefined relationships and can only be approximated; datum shift can reach kilometers if a point is far from the origin of one or both datums.

Did You Know?

• Mars has no oceans and thus no sea level, but at least two martian datums have been used to locate places there.
• Coordinates in NAD 83 can differ from NAD 27 by up to several hundred feet.