Cardinal Marks and Isolated Danger Marks: Different Warning Functions

Cardinal marks and isolated danger marks are different types of navigational marks. A cardinal mark indicates by compass direction which side has the safest or deepest water. An isolated danger mark, by contrast, marks a single danger with navigable water around it. Both warn of hazards, but they do not provide interchangeable information.

Cardinal Marks and Quadrants

The four cardinal marks are North, East, South and West. The direction identifies the safe side in relation to the mark: it is north of a North Cardinal mark, east of an East Cardinal mark, south of a South Cardinal mark and west of a West Cardinal mark. The mark therefore provides specific directional information, not merely a general warning of a nearby hazard.

Cardinal marks have black and yellow bands and two black cones as topmarks. Their orientation distinguishes the four directions: on the North mark, both cones point upwards; on the East mark, the cones are base to base; on the South mark, both point downwards; and on the West mark, the cones point towards each other. Direction, colour and topmark together make up the identifying features.

Isolated Danger Marks

An isolated danger mark is placed on or near an isolated danger, with navigable water all around it. It is black with one or more red horizontal bands. Two black spheres form the topmark. If lit, it displays a white light in groups of two flashes. The mark therefore indicates a single local danger, rather than—as a cardinal mark does—the safe side in a particular compass direction.

The warning not to pass close to an isolated danger does not specify a passing distance that is suitable in every situation. The appropriate passage in a particular area cannot be determined from the mark description alone. Charts and nautical publications for the area provide the local context. Navigational marks and their directional symbols form part of the marking of a channel, while the nautical chart provides the spatial context.

A Warning Function Is Not a Specific Passing Instruction

The type of mark tells you what information it conveys: cardinal marks indicate a side to the north, east, south or west; isolated danger marks identify a single obstruction with navigable water all around it. An overview of lighthouses, sectors and fog signals covers other visual and audible aids to navigation.