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Also called anti-grounding cone. This is an option on some ECS (and all ECDIS) to turn the COG predictor line into a cone so that it can then detect hazards that lie ahead.
It is a forward projecting cone assigned to the own ship (boat) icon that has a user defined angular width and extension. The extension is defined by a time interval multiplied by the speed over ground. qtVlm refers to the time interval as the reckoning time.
When the leading edge of this cone encounters a hazard it will sound an alarm or change to a red color. The hazards detected include the safety contour, or if none is set, the coastline contour, as well as any obstruction that is marked by an isolated danger symbol, such as rocks, wrecks, or other obstructions whose sounding is less than the safety contour.
As a rule, a danger cone does not trigger when crossing a normal aid to navigation.
The use of danger cones assumes that ENC are loaded, but they do not need to be showing. That is, you can have the ENC assigned and checked in the charts folder, but be viewing RNC, or no charts at all, and the danger cone will still function.