ABSTRACT
The basic unit of measurement for tidal calculations is metres and the information extracted from British Admiralty charts will also be in metres. It is important however, that the navigator always consults the Legend of every chart which confirms the units of measurements used for navigational and tidal data on that particular edition of the chart. A tide may be described as the regular and predictable movement of water governed by the relative positions of the astronomical bodies, primarily, the Sun and the Moon. The term refers to the time during which the tide comes in, normally over about a six hour period after Low Water until it reaches its maximum height above Chart Datum at High Water. Height of Tide is required to be calculated at any point in time in order to determine the more important value which is the Under-Keel Clearance between the keel and the seabed.
