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Go Back   SailNet Community > On Board > Gear & Maintenance > GPS Accuracy - how much is actually needed
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Topic Review (Newest First)
03-05-2014 07:38 PM
MarkofSeaLife
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!
03-05-2014 06:43 PM
pdqaltair
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.
03-05-2014 05:44 PM
TakeFive
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.
03-05-2014 05:27 PM
cthoops
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?
Yep. I don't know the mechanics behind it, but it's built into the iPad itself.
03-05-2014 04:41 PM
Group9
Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by findrichard View Post
Back in the summer of 2001 I was sailing solo up the east coast of the US, somewhere around NJ the fog set in as I was heading for a small inlet to pick up a mooring for the night. There was a marker at the mouth of the inlet with a bell on it that I could hear but could not see in the fog. Because of the fog, I could not determine what direction the sound of the bell was coming from. The night before, as was my custom, I put all the way-points for the next days sail in my handheld GPS. So I turned the GPS on and followed it directly toward the buoy. The buoy broke out of the fog at 12:30 off the bow at about 50 yards away. With map in hand I safely navigated past the rocky shoreline into the inlet where I was met by the harbor master's assistant who escorted me to my mooring for the night. Without GPS I would have turned seaward and spent another night on the open sea - something I really would not want to do in the fog. That little GPS was the best $150 I ever spent.
I remember getting caught out in the fog at dark in the 80's one time, and trying to find the pass through a spoil bank, using nothing but my depth finder and compass. One of the most miserable times I ever had in a boat. With today's gps/chartplotters, it would be so simple to get in when caught in that situation again.
03-05-2014 03:42 PM
TakeFive
Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by hellosailor View Post
Maine-
Let's see. With a chart scale of 1:25,000 and a common 0.5mm mechanical pencil, unsharpened, that pencil line would be 25,000 x 0.5mm wide in the real world. 12,500mm at 1000 mm to the meter, 12.5 meters wide.
But if you buy that mechanical pencil at a real supply store it can easily be 0.3mm lead, giving us 25,000 x 0.3mm, which will be 7.5 meters wide in the real world.
I'm not sure if you can sharpen a conventional pencil to a finer point.

Of course if the navigator is using a vintage 5/0 or 6/0 Rapid-O-Graph, they can do twice as well as that.

Personally I prefer the safety zone created by a nice dull Crayola Crayon. (G)

But back to the OP's question: Offhand, anytime you can increase accuracy or precision, that's a good thing. What they are seeing is a signal degradation caused by the hull and deck, and that probably will increase under heavy rain and cloud cover. Ten meters, twenty meters..."Good enough for government work."
I wish you guys were around when I took my ASA 105 test. I was trying to use similar arguments to explain why it was impossible for me to get the "exact" answer that the instructor was requiring of me. He seemed clueless about significant figures, loss of precision due to subtractive analysis, magnification of errors caused by long extrapolation, etc. I still scored 95%, but on the problems where he knocked off points, he seemed to expect me to shoot a golf ball with an arrow from a mile away.
03-05-2014 03:26 PM
findrichard
Re: GPS Accuracy - how much is actually needed

Back in the summer of 2001 I was sailing solo up the east coast of the US, somewhere around NJ the fog set in as I was heading for a small inlet to pick up a mooring for the night. There was a marker at the mouth of the inlet with a bell on it that I could hear but could not see in the fog. Because of the fog, I could not determine what direction the sound of the bell was coming from. The night before, as was my custom, I put all the way-points for the next days sail in my handheld GPS. So I turned the GPS on and followed it directly toward the buoy. The buoy broke out of the fog at 12:30 off the bow at about 50 yards away. With map in hand I safely navigated past the rocky shoreline into the inlet where I was met by the harbor master's assistant who escorted me to my mooring for the night. Without GPS I would have turned seaward and spent another night on the open sea - something I really would not want to do in the fog. That little GPS was the best $150 I ever spent.
03-05-2014 03:05 PM
hellosailor
Re: GPS Accuracy - how much is actually needed

Maine-
Let's see. With a chart scale of 1:25,000 and a common 0.5mm mechanical pencil, unsharpened, that pencil line would be 25,000 x 0.5mm wide in the real world. 12,500mm at 1000 mm to the meter, 12.5 meters wide.
But if you buy that mechanical pencil at a real supply store it can easily be 0.3mm lead, giving us 25,000 x 0.3mm, which will be 7.5 meters wide in the real world.
I'm not sure if you can sharpen a conventional pencil to a finer point.

Of course if the navigator is using a vintage 5/0 or 6/0 Rapid-O-Graph, they can do twice as well as that.

Personally I prefer the safety zone created by a nice dull Crayola Crayon. (G)

But back to the OP's question: Offhand, anytime you can increase accuracy or precision, that's a good thing. What they are seeing is a signal degradation caused by the hull and deck, and that probably will increase under heavy rain and cloud cover. Ten meters, twenty meters..."Good enough for government work."
03-05-2014 02:59 PM
RichH
Re: GPS Accuracy - how much is actually needed

.... and those charts are what is the base data on your chartplotter !!!!!!!

The GPS system did not draw those charts on your chartplotter; people with essentially 'crayons' and ink pens did.

;-)
03-05-2014 02:50 PM
JonEisberg
Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by RichH View Post
Quote:
Originally Posted by Maine Sail
Can one even draw a line on a chart, even with a very sharp pencil, that is less than 24' wide when adjusted for scale..?
Touché . Can one even enter a course-line to next waypoint or 'route line' on a chartplotter, even with a very high magnification, that is less than 24-100' wide?


Damn those stacked up 'tolerances' and 'resolution' problems.

;-)
Even more importantly, that applies to the cartographers creating the charts, to begin with...

From Nigel Calder's article cited above:

Quote:

The finest line that can be drawn is about 0.1 mm wide, but such a thin line is hard to see, and as such, it is not recommended, sometimes forbidden, to be used for drawing features such as coastlines and other critical objects. As a result, various hydrographic offices have adopted 0.2 mm as the finest line to be used on a chart. Let's say the hydrographer decides to plot the smooth sheet at a scale of 1:20,000 (in other words, one millimeter or inch on the smooth sheet represents 20,000 mm or inches on the ground). At 1:20,000, a line that is 0.2 mm wide represents 20,000 x 0.2 = 4,000 mm on the ground. This is four meters. Even if the survey is accurate to within inches, this plotting accuracy has now become the limiting condition in the accuracy of the final product. If the smooth sheet is plotted at 1:50,000, the plotting accuracy is ± 10 meters. If the pencil used to plot the data is not sharp and draws a line that is 0.5 mm wide, the plotting accuracy at 1:50,000 goes down to 25 meters, or 27 yards!
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