Revent/Key:datum

datum:horizontal=*
datum:horizontal:epoch=*

For vertical datums, see ele=*.

The datum and epoch (date) at which the given coordinates were measured, or to which they have been transformed.

These keys are mostly relevant for survey points, or imported datasets, which have a precisely surveyed location given on a specific geodetic datum.

As of January 3 2021 all coordinates measured by GPS, or traced from satellite imagery, are in WGS84(G2139) @ 2010.0. This coordinate systems differs from the "original" WGS84, which was tied to NAD83, by approximately 1-2 meters, depending on location.

When such datasets are imported to OSM (or survey points are mapped) the coordinates should, if possible, be transformed to the current realization of WGS84. This will allow them to match data created by other editors from GPS or satellite imagery.

Stating the specific datum and epoch will allow other editors, in the future, to transform this data to future realizations of WGS84.