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If Ordnance Datum was 'Mean Sea Level', then low tides should usually be expressed as negative vaues. They are not on tide timetables. So OD must refer to Low Spring Tide level.22.214.171.124 20:11, 12 November 2006 (UTC)
Tide tables are expressed relative to Chart Datum, not Ordnance Datum. Chart Datum is indeed based on Lowest Astronomical TideUKHO FAQ. jnestorius(talk) 21:23, 12 November 2006 (UTC)