Traditional or digital navigation - which is safer?

[jtsailjt]

Originally Posted by Bycrick

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Variation does change slowly, but in my example if you haven’t 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.



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Where/when has variation ever changed by 5 degrees since the invention of GPS plotters?
So that’s 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 don’t, so it’s not.

[S/V Adeline]

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)

[Benz]

Originally Posted by S/V Adeline

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.

[valhalla360]

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).

[Pizzazz]

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.

[Bycrick]

WRT the variation CHANGING five degrees suddenly, that isn’t 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 don’t try to tell me that programmers would never do something so stupid.

[S/V Adeline]

Originally Posted by Pizzazz

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.

[Pizzazz]

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?

[Bycrick]

The fact that you can "override" the magnetic model values is rather meaningless if you don’t 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 you’re 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 I’ve 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 it’s 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 don’t end up on the rocks.

[Pizzazz]

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?

[Bycrick]

The magnetic model is only a database. True. And it’s normally only good for five years. But last year they had to do a special update because it wasn’t judged sufficiently correct. Do you know what yours is?

If you’re cruising on a long passage and your predicted position differs from your GPS position, what assumptions can you make? How to you check them? Your answer seems to be just trust the electronics.

Navigation is always safer when you KNOW you have a problem. Because then, if you’re a prudent sailer, you’re trying to find out what and how bad the problem is. Being ignorant of, or ignoring, the problem may make you feel more comfortable, but it doesn’t make you any safer.

What kind of electronic compass? What effect will stray local magnetism have? Will it calibrate properly under all conditions? If it doesn’t calibrate, will it tell you? I have three electronic (flux gate type) compasses and a GPS compass. All from reputable manufacturers of "marine equipment." I’ll spend an afternoon calibrating and checking them all. I can seldom get them all within +/- 5 degrees, and the readings will vary depending on the course steered.

Comparing against a known radar heading? How closely is the radar dome and it’s lubber line marker set to the boat's line? The horizontal beam width on most 24” sailboat radars is +- almost 6 degrees. How close can you really estimate direction for a target?

Just because one uses electronic navigation doesn’t mean it plug-and-play or set-and-go.

[Benz]

Originally Posted by Pizzazz

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.

Radar is part of digital navigation. And radar won't show submerged rocks or reefs. There is a whole arsenal of non-digital means for the traditional navigator, not just a sextant. There is the sounding lead, the eyeball (which many people apply to an array of screens rather than to the outside world, and leads to their demise in many cases), the hand-bearing compass. When your DMA charts of the eastern San Blas islands seem to be completely out of kilter with the GPS, I guarantee that if you navigate by taking compass bearings on islands and features, and keep an eye out for coral, you can navigate safely. I've sailed the entire archipelago without turning on the GPS, and found the charts perfectly reliable, because I was piloting by the same means used to survey them originally, and everything fit together.

[Jim Cate]

If you’re cruising on a long passage and your predicted position differs from your GPS position, what assumptions can you make? How to you check them? Your answer seems to be just trust the electronics.

At the risk of being branded an unsafe navigator, I find the above criticism kinda silly. One's "predicted position" is, I guess, determined by DR... or do you have some magic means for making the prediction?

And DR at sea is influenced by a lot of factors well beyond control or measurement (except by electronic means). Leeway, which varies considerably with conditions and current set are pretty well guesses at best and can lead to quite substantial errors. Oh, don't forget that human helmsmen are pretty fallible in terms of adhering to a compass heading, no matter how accurate the compass is and h ow well you know the variation. Or if you are using a wind vane, as many serious cruisers still do, it will wander quite a bit, both following small wind shifts and its own idiosyncrasies. Auto pilots do a much better job of maintaining course, but they fall under the dread "electronic" category, don't they, and are prone to some vagaries themselves. In short, expecting DR positions to be accurate at sea is wishful thinking in many situations.

So, when your GPS and your predicted position disagree, one might check against another GPS. If they agree, why would you not believe them? Seems a lot more likely to be correct than any DR could hope to be. And, of course, out to sea, knowing one's exact position isn't very important... those of us who did use celestial were pretty happy with a cocked hat that would yield a < 5 mile error if conditions were sloppy.

Now, back to the changing variation. Even with old charts, in most of the world the errors will be on the order of a few degrees at the worst, and with consideration of the above paragraphs, that error will not cause much strife, ESPECIALLY if you check your position (electronically) a few times a day and make any correction in desired course that is needed. Monitoring your COG (electronically) gives rapid feedback and allows the navigator to alter course in a timely manner. This latter method is what we have used since the dawn of the satellite navigation era and it has served us well.

When in pilotage waters, augmenting the GPS derived position with visual observation and/or radar observation of coastal features is an easy checkup. And, in such waters, much closer attention to position is needed because of the presence of hazards the likes of which are not present at sea... no argument there!

Jim

[El Pinguino]

Originally Posted by Pizzazz

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.

I saw the thrust of the youtube vid as the fact that many people have no idea of the possible problems and errors that may exist within your electronic charting system. .... and that most nav courses do not address these problems.
Must be the glasses I am wearing.

Meanwhile... back in the dreamtime and the days of traditional navigation.
Jobbing navigators only used celestial navigation when 'off soundings'. Coastwise we navigated off the land and latitude and longitude were neither here... nor there.

The problem has arisen when 'modern' navigators have tried to blend the two.

Moving right along..
A few years ago I decided to move on from burglebrand CM93 and got the ISailor App together with a Chilean portfolio.
Wonderful stuff... the cartography was spot on...until I called at Puerto Barrosa... a new port to me..
Luckily I hadn't been lulled completely into a false sense of security... and we made port in daylight... the inward track is the one with the 'flat batt break'.... we actually anchored by the red '+'.

Just 'cos you bought it doesn't mean it is any good.....

[El Pinguino]

A certain lack of familarity with the Admiralty Chart seems to be manifesting itself in this thread.
Admiralty charts have the variation for a given year displayed on them.. on the coastal charts it is incorporated the compass rose... on ocean charts as 'magnetic variation curves'.
Also displayed is the rate of change per year.

In the example below the rate of change is 4' a year so a degree every 15 years or 3º since that chart was printed. The rate in some areas is far greater so 4 or 5 degrees since the dawn of GPS is not impossible.

NB That chart has been on the bulkhead at Oficina Pingo for 40 years and is not in use afloat.