Cardinal Marks are used in conjunction with the compass to indicate the direction from the mark in which the deepest navigable water lies, to draw attention to a bend, junction or fork in a channel, or to mark the end of a shoal.
Mariners will be safe if they pass North of a North mark, South of a South mark, East of an East mark and West of a West mark. Cardinal Marks are also used for permanent wreck marking whereby North, East, South and West Cardinal buoys are placed around the wreck. In the case of a new wreck, any one of the Cardinal buoys may be duplicated and fixed with a Radar Beacon (RACON).
At night, the lights of Cardinal Marks are programmed with distinct identifying characters; as an aide memoire they can be considered to flash in accordance with positions on a clock face whereby an East Cardinal flashes 3 times, a South Cardinal 6 times (but with an added long flash to make it more distinctive) and a West Cardinal 9 times. The North Cardinal doesn’t quite fit the pattern – having a continuous quick or very quick flash.
The buoy illustration shows Class Two configurations of buoys. These are approximately 3 metres in diameter and weigh approximately 6 tonnes excluding moorings. Buoys are needed to be recognised both in daylight and at night and use 'Top Marks' to assist in identification. A Top Mark on a Cardinal Buoy is triangular and coloured black. Top Marks and buoy colours themselves are arranged in order to represent the points on a compass.
Above (Left to Right): North, East, South and West Cardinal Class Two Solar Buoys