Geodetic reference systems
Geodetic reference systems describe how a coordinate system is embedded in the real Earth; they provide the theoretical foundations and define the key elements that are essential for determining coordinates:
- Origin of the coordinate system (zero point, the Earth’s centre of mass)
- Definition of the axes (e.g., the X‑axis runs through the intersection of the Greenwich meridian and the equator, the Z‑axis corresponds to the Earth’s mean rotation axis)
- Unit of measurement: metre
- Reference surface of the Earth (e.g., a rotational ellipsoid that approximates the Earth’s shape as closely as possible)
There are global, European and national reference systems that choose the appropriate characteristics according to the intended application, so that the system meets different needs. The most widely used is the World Geodetic System of 1984 (WGS‑84), which expresses positions in latitude and longitude and, in particular, serves as the basis for GPS. It is aligned with the International Terrestrial Reference System (ITRS).
Many countries have also defined their own national systems that are better suited to their territory. Switzerland is among them and has defined the globally referenced system CHTRS95 and a locally referenced system CH1903+. The transformation between the two systems can be performed by a simple translation. The official national coordinates are based on CH1903+, which can be converted to LV95 coordinates through a predefined projection.
In brief
The following elements form part of a reference system:
- the coordinate system, with its origin, axes and unit of measurement
- the Earth’s reference surface: the rotational ellipsoid and the gravitational field (geoid)
Global terrestrial reference systems
Global terrestrial reference systems are geocentric Cartesian coordinate systems whose origin lies at the centre of mass of the Earth M. A ‘Conventional Terrestrial System’ (CTS) is defined as any system whose origin lies at the centre of mass M, whose Z-axis passes through the North Pole (Conventional International Origin or CIO), whose XZ plane contains the mean origin at the Greenwich Observatory, and whose Y-axis completes the system to form a right-handed Cartesian coordinate system. The system is defined by the International Earth Rotation and Reference Systems Service (IERS) and specified in the IERS Conventions (2010).
Regional and local reference systems
Regional reference systems are defined for large-scale regions, e.g. the European Reference System for Europe, while local reference systems correspond to the adapted national coordinate and height reference systems. These are officially used for national surveying and official cadastral surveying in various countries worldwide. This category also includes local reference ellipsoids and map projections with their specific national coordinate systems.
European reference systems
In addition to the conventional national reference systems, there is a European reference system called ETRS89. The connection of national reference systems to global, terrestrial reference systems by means of geodetic measuring methods results in only a limited degree of accuracy. However, modern (local) reference systems, distinguish themselves by providing a precisely defined link to global systems, thus allowing absolute positioning within a centimetre.
Swiss reference systems
With national survey LV95 a global (CHTRS95) and a local (CH1903+) system were defined in Switzerland which have functioned as the official Swiss reference system since 2017. The latter replaces the old local reference system (CH1903), which formed the basis for surveying in Switzerland for more than a hundred years.
Federal Office of Topography swisstopo
Seftigenstrasse 264
3084 Wabern