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Old Today, 03:55   #76
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Re: Traditional or digital navigation - which is safer?

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Originally Posted by Bycrick View Post
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Variation does change slowly, but in my example if you havenít updated your plotter or GPS unit since you bought it, do you know if the conversion from true to magnetic is accurate? If it was off by 5 degrees when you set your AP, that might be important.



.
Where/when has variation ever changed by 5 degrees since the invention of GPS plotters?
So thatís not a very realistic thing to worry about. Maybe if someone had a 200 year old GPS chart plotter your scenario could be plausible, But they donít, so itís not.
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Old Today, 04:14   #77
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Re: Traditional or digital navigation - which is safer?

IMO, GPS is more accurate and therefore safer overall. But care and common sense should be applied in navigation.
Regardless of how accurate it is, our entire trip from Norfolk to the Outer Banks on the ICW GPS had our position approximately 100ft or so *starboard*, placing us on land for the majority of the trip. (Three separate GPS units, for those who wish to condemn the unit, I believe it is an issue with chart overlay)
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Old Today, 04:43   #78
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Re: Traditional or digital navigation - which is safer?

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IMO, GPS is more accurate and therefore safer overall. But care and common sense should be applied in navigation.
Regardless of how accurate it is, our entire trip from Norfolk to the Outer Banks on the ICW GPS had our position approximately 100ft or so *starboard*, placing us on land for the majority of the trip. (Three separate GPS units, for those who wish to condemn the unit, I believe it is an issue with chart overlay)
GPS can accurately pinpoint your position on the geoid--the model of the earth it's set for, but that doesn't mean the chart is aligned with the geoid. A lot of the world still needs to be re-surveyed and aligned with whatever the model is now (still WGS-84?). In many places it is, and you can feel around in a fog with safety. In many places it's not, and the GPS position has to be treated with suspicion there. I think people run into trouble when they blindly believe that the chart is aligned perfectly with the geoid, and that's when they pile up on the reef next to the entrance channel.
So it is safer overall only when everything lines up right; it is more dangerous when it fosters carelesness and a false sense of security.
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Old Today, 05:31   #79
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Re: Traditional or digital navigation - which is safer?

This subject seems to come up a couple times per year.

The flaw is always to assume that someone using electronic systems has no understanding of navigation and therefore is a danger. Guess what, the vast majority of professionals use electronic as their primary and may or may not use paper as a backup.

A lot of the issues are easily addressed in the modern world. Example: If you are worried about losing your GPS/Chartplotter, you can download a free app to each of your phones and have multiple backups completely independent of the primary. While it's possible they could all go down, it's starts straining credibility and goes off into the realm of wild what-if scenarios...you can always find one that can fail (including using paper charts and sextants).
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Old Today, 07:42   #80
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Re: Traditional or digital navigation - which is safer?

The original question was, what is safer, paper or electronic. Then the question of GPS alignment to the geoid to reality comes up and someone makes the claim that electronic navigation is not safer because of that. Ridiculous. How would you know, using paper charts and a sextant that the chart was offset 100 ft or how would you compare your sextant position to your radar bearings?

As discussed, electronic navigation all the way + radar overlay to confirm if you are paranoid.
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Old Today, 07:52   #81
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Re: Traditional or digital navigation - which is safer?

WRT the variation CHANGING five degrees suddenly, that isnít what I said or what I meant. My point was that electronic navigation devices use a magnetic model to convert from true to magnetic. The model is only valid for a certain time period (epoch). What does your device do when the installed model is out-of-date? Does it continue to use the old model or does it quietly quit using invalid data, thus potentially producing substantial errors?

Please donít try to tell me that programmers would never do something so stupid.
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Old Today, 08:11   #82
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Re: Traditional or digital navigation - which is safer?

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The original question was, what is safer, paper or electronic. Then the question of GPS alignment to the geoid to reality comes up and someone makes the claim that electronic navigation is not safer because of that. Ridiculous. How would you know, using paper charts and a sextant that the chart was offset 100 ft or how would you compare your sextant position to your radar bearings?

As discussed, electronic navigation all the way + radar overlay to confirm if you are paranoid.
My whole point was that while GPS is highly accurate, the chart overlay may or may not be so precise. When my vessel is mid channel and all three GPS navigation maps show me 100ft up on dry land, there is an issue. These are electronic charts, two land platforms , one marine platform, and one handheld unit that just provided latitude and longitude. All four gave the exact same latitude and longitude, but the three with electronic maps all showed the same position on the map: approximately 100ft OUTSIDE the water channel for about 20 miles. If it were dark or heavy fog that could be an issue, but paper would be irrelevant either way.
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Old Today, 08:15   #83
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Re: Traditional or digital navigation - which is safer?

The magnetic model is used to convert measured magnetic bearings by the compass to True headings that are useful for navigation. If your magnetic model gets out of date, you will immediately see the difference in COG vs Heading vectors. So you just align them or you just steer by COG. You can also manually override the model. What would you do with a paper chart and a sextant?
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Old Today, 09:31   #84
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Re: Traditional or digital navigation - which is safer?

The fact that you can "override" the magnetic model values is rather meaningless if you donít know enough to realize that something is wrong and it needs to be overridden. Is the difference between my magnetic compass and my COG from my GPS reading caused by a current, or increased leeway or because my wife moved the sewing machine too close to the AP compass? Where is the Variation value in your nav system coming from? The GPS? The MFD? Have you updated the software? In all the units? If you have multiple MFDs or electronic compasses with built in magnetic models are they all using the same data? Of course if youíre careful you CAN check and verify the data. But do you? When was the last time you swung your steering compass? Or compared all of your compasses against a known heading?

As to the statement about the change in variation when using paper charts, every marine chart that Iíve ever seen has a printed compass rose with both True and Magnetic bearings shown. It also says what the variation was when the chart was printed and gives an estimate of the annual change. I challenge you to tell me anything about the magnetic model in your electronic system.

Arguing the details of the magnetic model is simply trying to sweep the general problem under the rug. Does someone know enough to know what their chosen nav system is doing FOR them versus what itís doing TO them. Neither traditional nor electronics systems are always accurate all the time. To assume that they are is placing blind faith in something that you know has mistakes and errors and then hoping you donít end up on the rocks.
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Old Today, 14:53   #85
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Re: Traditional or digital navigation - which is safer?

The magnetic model is just a database (table) with estimated variations by location. It is far more accurate than the table printed on the outdated paper chart. With electronic navigation, if COG and heading deviate, you would know you have a problem. With a paper chart and magnetic compass you would never know there was a problem. There could be many sources of the problem which require different solutions but how could navigation be safer if you would not even know there were a problem?

By the way, this is why I do not have a steering compass anymore but I have some really good, self adjusting electronic compasses. How often do I calibrate those? Well, one self calibrates, the other one takes two circles and I do check them in flat waters against a known radar heading approximate once a month. On the other hand, cruisers calibrate their magnetic compasses may be once every five years?
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