referencessystems

Reference systems, or coordinate reference systems, provide the framework for specifying positions, orientations, and motions of objects within a given space. They combine a mathematical space (a coordinate system), a measuring unit, and a reference surface or datum that anchors coordinates to the real world. Reference systems can be spatial, temporal, or both, and are essential for navigation, surveying, mapping, astronomy, and physics.

Spatial reference systems include geographic coordinate systems that use latitude, longitude, and sometimes altitude, typically tied to geodetic datums such as WGS84, NAD83, or ETRS89. They also encompass geocentric or Earth-centered frames like ECEF (Earth-Centered, Earth-Fixed) and inertial frames used in astronomy (ECI). Map projections convert the curved Earth to a plane and yield specific coordinate reference systems for maps and GIS data (for example, UTM zones or Web Mercator). In GIS work, the term CRS (coordinate reference system) covers both the datum and the projection method.

Temporal reference systems define time scales and synchronization, including UTC, TAI, UT1, and various astronomical time scales. Time-referenced frames are crucial for precise navigation and astronomy, and systems such as GPS provide time along with positioning.

In physics, reference frames can be inertial or non-inertial, and the choice of frame influences the description of motion and forces, as described by classical mechanics and relativity. Standards and conventions underpin reference systems, with organizations like ISO and IERS maintaining frameworks for geodetic positioning and timekeeping. Common examples include WGS84, NAD83, ETRS89, and the International Terrestrial Reference Frame (ITRF).