Bob-
"Admiralty
charts to be transferred to WGS 84 datum, says U.K. Hydro Office
Sea Technology, Mar 2000
E-mail Print Link The United Kingdom Hydrographic Office (UKHO) will start a three-year programme this month to transfer 310 Admiralty
charts of Great Britain to a horizontal datum compatible with WGS 84 Datum.
The transfer of the remaining
charts of the British Isles will follow. Currently, Admiralty
charts of the British Isles are referred to several regional datums, for example Ordnance Survey of Great Britain 1936 (OSGB 36) Datum. However, navigational positions obtained from satellite positioning systems, such as the global positioning system (GPS), which are now used for navigation, are normally referred to World Geodetic System 1984 (WGS 84) Datum. "
Admiralty charts to be transferred to WGS 84 datum, says U.K. Hydro Office | Sea Technology | Find Articles at BNET.com
So apparently Admiralty Chart Datum (ACD) is known to be offset from WGS84 and should not be expected to agree with
GPS positioning. Did Navionics ever give you an answer?