Isolated Danger marks. Isolated Danger marks are erected on, or moored on or above, isolated dangers of limited extent which have navigable water all round them. The extent of the surrounding navigable water is immaterial: such a mark can, for example, indicate either a shoal which is well offshore, or an islet separated by a narrow channel from the coast.
Note (b) : The double sphere topmark is a very important feature of every Isolated Danger mark by day, and should be used wherever practicable and be as large as possible with a clear separation between the spheres.
On Admiralty charts, the position of a danger is the centre of the symbol or sounding indicating the danger. The symbol indicating the Isolated Danger buoy will inevitably be slightly displaced.
Topmark. Black double-sphere topmarks, disposed vertically, are a very important feature of Isolated Danger marks and are carried whenever practicable.
Colours. Black with one or more red bands are the colours used for Isolated Danger marks.
Shape. No significance is attached to the shape of Isolated Danger marks, but in the case of a buoy, a pillar or spar buoy is used.