Transforming between WGS84 Realizations

Abstract

The World Geodetic System 1984 (WGS84) has been consistent with the International Terrestrial Reference Frame (ITRF) since 1994. After the original release of WGS84 in 1987 by the Defense Mapping Agency (since superseded by the National Geospatial-Intelligence Agency, NGA), there have been six subsequent realizations of the frame, designated WGS84 (G730), WGS84 (G873), WGS84 (G1150), WGS84 (G1674), WGS84 (G1762), and WGS84 (G2139). Users of WGS84 often fail to document metadata indicating the realization and epoch related to their data collections. As a result, when transforming between realizations or between other reference frames, the transformation can lead to incorrect results. We aim to bring awareness to the nature of WGS84 and the ambiguity in how it is defined. An internally consistent set of transformation parameters is derived that allows for transformation between the WGS84 realizations and other global reference frames while accounting for nuances and inconsistencies in the published literature. The derivations are based on existing (though not easily discoverable) information. Transformation accuracy is evaluated through comparison to other software and NGA-published GPS tracking station coordinates. After accounting for plate tectonic motion, the estimated uncertainties average ±0.03 m horizontally and ±0.08 m vertically for transformations involving all realizations other than the original WGS84. Uncertainties are at the meter level for transformations that include the original WGS84. If plate motion is ignored, the horizontal uncertainties can increase by several decimeters, depending on location within a plate.