A cardinal mark is a sea mark (a buoy or other floating or fixed structure) used in maritime pilotage to indicate the position of a hazard and the direction of safe water.
Cardinal marks indicate the direction of safety as a cardinal (compass) direction (north, east, south or west) relative to the mark. This makes them meaningful regardless of the direction or position of the approaching vessel, in contrast to the (perhaps better-known) lateral mark system.
The characteristics and meanings of cardinal marks are as defined by the International Association of Lighthouse Authorities.
A cardinal mark indicates one of the four compass directions by:
The topmark configurations for north and south are self-explanatory (both cones pointing up, or both pointing down). Those for east and west "follow the Sun"—the top cone points in the direction in which the rising (for an east mark) or setting (for a west mark) Sun appears to move with respect to the horizon, while the bottom cone points in the direction in which its reflection on the ocean surface appears to move. (An alternative memory aid is to see cones pointing inwards as a waist ("west"), and cones pointing out forming an egg ("east").
In the diagram, the light sequence is indicated by the white-banded pattern and the notation Q (for quick) or VQ (very quick). Either the quick or the very quick sequence may be used; the choice allows for two similar nearby marks to be uniquely identified by their lights.
A cardinal mark may be used to accomplish the following: