World Geodetic System 1984 (WGS84)

WGS84 is the geocentric reference datum and ellipsoid used by GPS and by ECDIS electronic charts; its semi-major axis is 6,378,137 m and flattening 1/298.257223563. Positions on a WGS84 chart match GPS output directly, so no datum shift is applied. Older paper charts on local datums can differ by tens to hundreds of meters from WGS84, and the chart’s title block carries the shift to apply before plotting a satellite fix. The datum was last refined in successive realizations to align with the international terrestrial reference frame.