FOM-3.17	Fleet Operations Manual Navigation Chart Datum	Doc No.: FOM-3.17 Revision: 01 Date: 15 Oct 2024 Issued by: DPA Approved by: MD

1. CHART DATUM

• GPS positions are referenced to the World Geodetic System 1984 Datum (WGS 84). This may not be the same as the horizontal datum of the chart in use. The receiver may convert the position to other datum.
• In this case, the observers must ensure that they are aware of the datum of the displayed position.
• DGPS positions are normally referenced to WGS 84, though regional datums, corresponding to WGS 84, may be used [e.g. North American Datum 1983 (NAD 83) in the USA] and European Terrestrial Reference System 1989 (ETRS 89).
• Many areas of the world have not been surveyed to modern standards; hence, the positional accuracy of the charted detail on the paper chart, Raster chart, or ENC may not be as accurate as the GNSS (Global Navigation Satellite System) receiver-derived position.
• Navigating Officers must read the note on satellite-derived positions on the Admiralty charts for more information.
• Further information can be found in the Mariner’s Handbook (NP 100) and in Annual Summary of Admiralty Notices to Mariners, No. 19.
• Volume 2 of The Admiralty List of Radio Signals published by UKHO contains full descriptions of all GNSS systems, with notes on their correct use and limitations. Also included are descriptions and examples of over-reliance on GNSS, and a full account of the problems caused by differing horizontal datums.