I subscribe to the "four missed meals from anarchy" theory of seamanship, I suppose. I will happy use GPS as it is available, but I will make it one block in a foundation consisting of celestial, pilotage, chart work and plain old "does this look right to you?" or the instinctual input fostered by using compass, pelorus, landmarks, sea state, the look of the sea near river deltas you can't see, the look of the sea near reefs, presence of birds, flotsam in the water...a whole array of evidence that will alert me to the fact I may not be exactly where I think I am.
The day some years ago when, despite having a five-satellite lock while I was driving a straight course at four knots, I saw the GPS briefly clock me at 60 knots SOG until my position was "recalculated" at a NM southwest of where I had reportedly been five seconds previously, was the day I decided my faith in this technology should be somewhat less than total.
Prudent seamanship relies on being conversant with a wide range of skills, techniques and even habits of mind, some of which fall outside of rational analysis. Because the GPS system has no stake in whether it's knocked out of orbit by solar storms or if its receivers get a free trip to the sea bed, and I do, I reserve the right to take its little numerical opinions with a grain of sea salt.
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