Datum

A geodetic datum is a reference system that forms the foundation for all measurements and mapping related to the determination of the Earth's geometric shape, size, gravity field, and temporal variations. A datum defines the reference surfaces or origin points for measurements, thereby enabling the construction of coordinate systems, height systems, and gravity systems.

Types

Geodetic datums are classified according to the dimensional attributes they define:

• Horizontal datum: A coordinate system established for determining the horizontal positions of points on the Earth's surface. It is defined by the horizontal coordinates of geodetic control points and is realized through horizontal control networks.

• Vertical datum: A reference system used to determine the elevation of surface points. It is realized through vertical control networks and is typically referenced to a physical surface, such as mean sea level.

• Three-dimensional datum: A system capable of defining both horizontal and vertical positions of points simultaneously. It is realized using space geodetic techniques, including GNSS, VLBI, SLR, and DORIS. The International Terrestrial Reference Frame (ITRF) is a prominent example of such a datum.

• Gravity datum: A reference system based on gravity measurements. It is supported by gravity control networks and is employed in physical geodesy. An example is IGSN71, which serves as a global gravity datum.

• Sounding datum: A vertical reference surface used in hydrographic charts to measure water depths. It is typically defined by long-term observations, such as the lowest astronomical tide (LAT).

Applications

Geodetic datums are indispensable in fields such as cartography, civil engineering, navigation, remote sensing, and geographic information systems (GIS). In all these applications, referencing data to the same datum is essential for ensuring consistency, comparability, and integration across different time periods and regions.

Datum Transformations

To compare data obtained in different datums, coordinate transformations are necessary. These transformations typically involve parameters of position, orientation (azimuth), and scale. Modern systems, such as GPS, operate directly within Earth-centered, global datums (e.g., WGS84, ITRF), requiring transformation parameters to ensure compatibility with local mapping systems.

Modern Developments

Today, classical horizontal and vertical datums are generally regional in nature, whereas three-dimensional and gravity datums possess global applicability. Furthermore, the development of Continuously Operating Reference Stations (CORS) has enabled the establishment of dynamic datums that provide high-accuracy, real-time, and temporally updated positional data.