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Old 03-04-2014
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Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by Frogwatch View Post
Good God! If you need your GPS to be more accurate than 1000' you are doing things wrong. If you are in a situation needing better than that you better be using sights on objects using a hand compass cuz GPS can be seriously wrong at times. If you cant take sights then you shouldnt be there or should be dead stopped until visibility gets better. A prudent navigator relies on what he sees, not something a black box tells him.
For those who say charts are wrong, well, even with gps they would be wrong and you are still screwed.
OK, fishermen need gps to get coords of good spots and I use mine to give me good speed measurements and a position measurement when out of sight of land but I tend to rely more on my dead reckoning first and if they don't agree, somethings wrong.
In 27 yrs of extensive sailing, I've never needed "waypoints" and still dont get why people do. Instead, for example exiting Sea of Abaco near Little Harbor, I get to where the "white house" lines up at X degrees and follow the reciprocal course until my GPS Lon. reads zz.zzz to indicate that I am well clear of all the hazards. Then I can head on a course of 170 to Spanish Wells. Waypoints? Why?
Even going from Marsh Harbor down to Little Harbor where there are numerous turns, reefs and other hazards, I better be using my compass to sight on known points cuz that also alerts you to stuff that isn't on the chart. Why would waypoints help?
Why waypoints? Well for starters, if you are crossing a body of water where you can't see the landmarks to guide you to your destination using the gps waypoints will allow you to maximize your VMG by compensating for current, leeway, etc.
Why accuracy greater than 1000'? I can think of two specific examples off the top of my head. Once we were navigating a stretch of water that was a natural channel with bands of reefs on both sides. We were looking for the gap in the reef that would allow us to get out of the channel and shave several hours off our trip. Our gps plotter showed us exactly where to look for the gap, which was very difficult to spot from a distance. The gap was exactly where the gps said it would be. Another time we were coming home, and as we approached the city the entire bay was engulfed in heavy fog. The gps told us exactly where we were in relation to the shipping lanes and the mud flats, so I didn't have to worry about that, and was able to devote 100% of my attention on watching for sport fishing boats and anchored freighters.

A gps is a valuable tool, but only if you can trust it's accuracy. I always double check with visual references when I can, and I have come to trust what the gps tells me when I can't see.

Of course there will always be those people who want to do it the old fashioned way with paper charts, but in my opinion the technology is reliable enough that the paper charts serve only as a back-up.
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