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  #41  
Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by Frogwatch View Post
cthoops:

What if you are out of cell or wifi range? Do you still have GPS?
One of the greatest accomplishments of corporate misinformation has been the cell companies' success at making people think that you need cell service to have have GPS. *

While out of cell or wifi range, you should be sure to use a chartplotter program that caches the charts locally. Most of them do that, but some of the freebies do not.


* Another similar piece of misinformation is that you need a "High Definition" cable plan to receive local broadcast channels in Hi Def. In virtually all metropolitan areas, all you need is an ordinary old-fashioned antenna. All the local channels are REQUIRED to broadcast in high definition.
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  #42  
Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by RichH View Post
NO,
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.
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  #43  
Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by Frogwatch View Post
OK, OK, I am almost convinced that chart plotters are nice. After considering the 4-5 times I have been really confused about exactly where i was I can see that a chart plotter would be nice.
When I was 17, in the late 1970s, I had been sailing for ages in Sydney harbour, but the big stuff was the long offshore races including the Sydney Hobart. I pleaded with an owner to take me offshore and my first step onboard a huge offshore racer was a day of nerves and excitement. Well, just before dawn on the second day of that first race the skipper ( a real god to me) and the navigator (second god) stood crustily at the nav station looking out the windows at the dim coast line just appearing. This was their conversation:
"Whats that headland?"
"Hmmmm, looks like Cape X"
"Or could be Cape Y"
"Yes, but it looks like Z Headland"

They didnt know where the f we were!

And this happened every bloody morning of every bloody race. We were lost most of the time!

Theres an old joke about the Sydney Hobart yacht Race: go out Sydney harbour, turn right, count 22 light houses and turn right again. But it wasn't a joke. Thats the way they did it!

So there's no looking back for me because I KNOW how navigators couldn't!
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