WGS 1984 is not what you think!

Description

In the European Petroleum Survey Group (EPSG) database, the World Geodetic Survey 1984 (WGS 1984 wkid 4326) is not one single geographic coordinate reference system. Instead, it is an 'ensemble' or bucket, that contains all definitions or realizations of WGS 1984. Each realization, or version, of WGS 1984 is specified by a reference to the GPS week for which it was calculated.

For example, the current realization in 2025 is WGS 1984 (G2296). Each realization (G730, G873, G1150, G1674, G1762, G2139, G2296) are all defined based on the locations of a set of control points on a specific date. That date is referred to as the epoch for the datum and geographic coordinate reference system.

A new WGS 1984 realization reflects the Earth's crustal motion changes over time, including tectonic plate or seismic movement, post-glacial rebounding, or subsidence.

Because GCS_WGS_1984 is not one single definition, a user cannot transform from WGS_1984 to WGS_1984_(G1762) for example, because WGS_1984 is not a single thing. To access a specific transformation going TO WGS_1984_(G1762), the user must specifically define the starting point, whether they are starting from WGS_1984_(G730), from WGS_1984_(G1150) or some other realization of WGS_1984.

Depending on the starting point, several transformations may be required to get to the desired one. For example, if the original data uses WGS84(G1674), three successive transformations are used to get to WGS84(G2296):

1. WGS_1984_G1674_To_G1762_Fixed_At_2012.5_KD
2. WGS_1984_G1762_To_G2139_Fixed_At_2016.0_NGA
3. WGS_1984_G2139_To_G2296_Fixed_At_2024.0_NGA

Cause

The latest lists of the components of the Projection Engine can be viewed and downloaded from the following article: Coordinate systems, map projections, and transformations

On the list of Geographic Datum Transformations, scroll to page 77 for the list of transformations between the realizations of WGS 1984.

To apply these transformations, the user needs to know which realization of WGS 1984 they are using and have that specific definition applied to their data to call the appropriate transformation on the projection engine.

Solution or Workaround

*Data from: Transforming between WGS84 Realizations

**Data from: Thirty Years of Maintaining WGS 84 with GPS, Section 2.2 Alignment Through PPP, and WGS 84 (G2296) Terrestrial Reference Frame Realization

WGS 1984 Epoch Transformations