Definition
A pre-computed vertical or horizontal sextant angle sets a boundary. For a vertical danger angle, the angle to a lighthouse of known height increases as the vessel closes; keeping the observed angle less than the pre-set value keeps the vessel outside a circle around the danger. Horizontal danger angles between two marks work identically, the angle growing as the vessel approaches. Both give continuous warning without plotting and remain valid regardless of compass error.
Want to apply this on the water?
Ask Saily out loud for an example, a scenario or a simpler explanation.
Talk to Saily