Using GPS to clear an isolated danger.
South west of Alderney in the Channel Islands there is a very dangerous rock called Pierre al V'raic. Although there are well established clearing lines that can be used in daylight to avoid it, people still occasionally hit it.
One reason may be because they set up the clearing lines then the visibility decreases and they can no longer use them, another is that when approaching from the south, the clearing lines may that have been selected may not be clearly visable in sufficient time.
To prepare for this possibility a waypoint can be placed on the rock. It is then a simple matter to monitor that waypoint's distance off and ensure it does not fall below an acceptable margin for error.
Essentially this gives you a guard zone around any danger.
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