A cardinal mark is a sea mark (a buoy or other floating or fixed structure) commonly 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:[1]
(always attached)
(if light attached)
Abbreviated as one of:
Either a quick or a very quick sequence of light flashes 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:
Other uses:
The north and south topmarks are self-explanatory (both cones pointing up, or both pointing down). Remembering the east and west marks can be more of a problem.
The most natural way to distinguish the topmarks for east and west is to "follow the Sun"—the top cone indicates the direction the Sun would appear to move when visible in the part of the sky that corresponds to the mark's communicated direction, while the bottom cone indicates likewise for the Sun's reflection on a calm ocean surface. So when a topmark consists of two cones pointing away from each other, this should suggest the Sun moving away from its reflection on a calm ocean surface, which happens when the Sun rises, and a rising Sun is visible to the east. Likewise, a topmark with two cones pointing toward each other should suggest a setting Sun, which is visible to the west.[3]
Other mnemonics have been devised for the east and west marks in case the mnemonic that uses the Sun is difficult to grasp. For example:
The colours can be remembered this way: The two conical top-marks always point to black.
