GPS Accuracy - how much is actually needed - Page 4 - SailNet Community
Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

Keep in mind that the location your GPS is telling you has a random deviation of up to 3-5 meters at any given time (from the signal, not the GPS) so your reading could vary up to 10 meters from the same spot at different times.
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Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

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..?

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Maine Sail is offline
Old 03-05-2014
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Re: GPS Accuracy - how much is actually needed

Quote:
Originally Posted by Maine Sail View Post
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.

;-)
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Old 03-05-2014
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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 Â± 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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Old 03-05-2014
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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.

;-)
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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."
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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.
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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.

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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.

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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?
Yep. I don't know the mechanics behind it, but it's built into the iPad itself.

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