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Terrestrial Reference Systems and Frames

Terrestrial Reference Systems and Frames

In this chapter of the summary, we will highlight some examples of Coordinate Reference Systems and Frames:

Terrestrial Reference Systems
Figure 1: Terrestrial Reference Systems

ITRS: International Terrestrial Reference System uses the GRS80 ellipsoid, with as its origin the centre of mass of the Earth. Greenwich (UK) is used as a meridian. When using time-dependent coordinates, you always need to specify the epoch (e.g. @2024.91).

ITRF: International Terrestrial Reference Frames are realisations of the ITRS. They publish updates every ~1-6 years. Due to the movement of tectonic plates, the differences between each iteration can be multiple centimetres to decimetres. Two examples are ITRF2014 and ITRF2020 (NB: Frame != epoch, e.g. ITRF2014@2022.90). These updates reflect:

  • Improved precision of the station positions \(r\)(\(t\)0) and velocities \(r\) due to the availability of a longer period of observations, which is particularly important for the velocities,
  • Improved datum definition due to the availability of more observations and better models,
  • Discontinuities in the time series due to earthquakes and other geophysical events,
  • Newly added and discontinued stations,
  • Occasionally a new reference epoch \(t\)0.

Changes in ITRF2014
Figure 2: Changes in ITRF2014

WGS 84 is aligned with ITRS (WGS 84-G2296 (2024) = ITRF2020). For time-dependent coordinates, however, it has limited precision. The realisations (frames) often have differences between m – cm. The ensemble code (+- 2m): EPSG:4326 for 2D (often used as unknown latlon).

ETRS89: European Terrestrial Reference System 1989 is ITRF89 at epoch 1989.00. It moves with a stable part of Europe, “no” time-dependency. A new realisation (frame) comes with every ITRF. ETRF2000 is recommended for mapping and the ensemble code has an accuracy of +- 0.1m.

A transformation from ETRS89 to ITRS (and WGS84) is an example of a Time-dependent transformation (2.4 cm/year), but a specification of realisation and epoch is needed and most software does not include this transformation. A Null transformation is possible, simple and the current practice, but it is not future-proof, since the difference is growing (now 0.9m).