Join Date: Nov 2008
Location: Deale, MD
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Re: GPS Accuracy - how much is actually needed
GPS (chartplotter) 'accuracy' is totally dependent on 'resolution accuracy' of the original charting from which it is based. Problem is that for most areas that do NOT have large scale commercial marine traffic, the base chart 'may' still be based on 1927 or earlier surveys. Plus and with respect to 'resolution errors', using GPS charting based on 'old' lead-line surveys may also induce EXTREME errors especially when the GPS chartplotter MAGNIFICATION greatly exceeds the 'magnification' of the original hand surveyed charting.
FWIW .... most USA marine charts are only upgraded if and only if there is large commercial boats constantly using the area of the chart.
OEM Charting error trumps GPS accuracy; 'over-magnification' by chartplotters MAGNIFIES the original errors.
I've found disagreements between paper and electronic charts up to one mile, and depth errors of 6 vs. 120 feet. And as Jack pointed out, they were remote areas.
In areas with rocks, I get it. In areas with sandbars, eyes top charts with surprising frequency.
And there is the guy who entered the coordinates of a ship channel marker as a way point; it lined-up behind the mast and he tagged it within the 4' accuracy bar. Plenty close enough with a cat.
(when asked how he reached the starting holds on a difficult rock climbing problem that clearly favored taller climbers - he was perhaps 5'5")
"Well, I just climb up to them."
by Joe Brown, English rock climber
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