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Old 09-06-2018, 09:05   #76
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Re: true vs magnetic bearing

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Originally Posted by Wotname View Post
Neither are entirely correct and both refer only to ships etc. AFAIK, there is no way to alter natural magnetic anomalies in / on the earths surface (above or below water).
Went a steel wrench left on a wooden mine sweeper, by mistake, can be detected at a degaussing station they must use something magnetic on the bottom. Something to consider.
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Old 09-06-2018, 11:21   #77
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Re: true vs magnetic bearing

I need to point out you are possibly confusing the different corrections that need to be performed on a compass. First you have to correct your compass deviation caused by metallic parts of your boat that affect the instrument itself Traditional compasses that use a magnet are the most effected and need to be corrected while Flux gate compasses generally require not deviation correction. Once corrected if you are sailing in local waters you can plot your course using magnetic north but all charts and maps are based off of true north. The correct way to set a course bering is to correct to true north by including the declination from magnetic north to true north in your course calculations. If you have a modern map platter it will automatically make that correction for you but if you use paper you will have to make that calculation. The difference in declination can be pretty significant and magnetic north is always on the move so the declination changes all the time. from place to place the changes are more significant. If you are in Miami the magnetic declination between the true north pole and magnetic is a correction of -6 degrees, 44 min. But if you are in New York and the Chesapeake the correction is -12 degrees, 48 min. Since the declination gets greater as you approach the magnetic pole by the time you reach Nova Scotia you are now at a declination of -17 degrees 39 min. Once you get to a variation of 20 degrees +/- normal magnetic and flux gate compasses are considered quite useless and you have to rely on non-magnetic based compass systems like a sun compass of gyroscopic compass to give you a heading using the earths axis as the reference point.
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Old 09-06-2018, 11:40   #78
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Re: true vs magnetic bearing

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Originally Posted by Wayfarer1008 View Post
I need to point out you are possibly confusing the different corrections that need to be performed on a compass. First you have to correct your compass deviation caused by metallic parts of your boat that affect the instrument itself Traditional compasses that use a magnet are the most effected and need to be corrected while Flux gate compasses generally require not deviation correction. Once corrected if you are sailing in local waters you can plot your course using magnetic north but all charts and maps are based off of true north. The correct way to set a course bering is to correct to true north by including the declination from magnetic north to true north in your course calculations. If you have a modern map platter it will automatically make that correction for you but if you use paper you will have to make that calculation. The difference in declination can be pretty significant and magnetic north is always on the move so the declination changes all the time. from place to place the changes are more significant. If you are in Miami the magnetic declination between the true north pole and magnetic is a correction of -6 degrees, 44 min. But if you are in New York and the Chesapeake the correction is -12 degrees, 48 min. Since the declination gets greater as you approach the magnetic pole by the time you reach Nova Scotia you are now at a declination of -17 degrees 39 min. Once you get to a variation of 20 degrees +/- normal magnetic and flux gate compasses are considered quite useless and you have to rely on non-magnetic based compass systems like a sun compass of gyroscopic compass to give you a heading using the earths axis as the reference point.
Thanks for that, Wayfarer... I grew up and studied navigation long before the days of plotters and fluxgate compasses. I learned from paper charts with the declination printed on the compass rose along with the date and annual variation from year to year.
Up until a few years ago when I ‘swallowed the hook’, I still used a paper chart because I didn’t trust the newfangled electronical instrumentation.
Cheers, Phil
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Old 09-06-2018, 12:43   #79
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Re: true vs magnetic bearing

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Thanks for that, Wayfarer... I grew up and studied navigation long before the days of plotters and fluxgate compasses. I learned from paper charts with the declination printed on the compass rose along with the date and annual variation from year to year.
Up until a few years ago when I ‘swallowed the hook’, I still used a paper chart because I didn’t trust the newfangled electronical instrumentation.
Cheers, Phil
Phil, nothing wrong with that. At least you know how to get home. Not sure about those relying on electrics entirely if the power goes out or one interlocked component takes the rest out. Nothing wrong with the electronics as long as they have paper backup.
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Old 09-06-2018, 13:30   #80
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Re: true vs magnetic bearing

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Canada owns the North Magnetic Pole not the north geographic pole. When using the North Magnetic Pole as a navigational reference is finally terminated because modern technology has rendered it obsolete (hopefully in the very near future) Canada will only own another piece of artic wasteland. Just kidding, the North Magnetic Pole has served us admirably for centuries. My thanks to Canada for letting us use it. Let's hope Canada erects a monument to it when it's no longer needed.
It was mentioned earlier, but the North Pole left Canada about 20 years ago, sorry. (and it moves faster every year)
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Old 09-06-2018, 13:41   #81
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Re: true vs magnetic bearing

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It was mentioned earlier, but the North Pole left Canada about 20 years ago, sorry. (and it moves faster every year)
Is it Armageddon? :biggrin
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Old 09-06-2018, 14:01   #82
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Re: true vs magnetic bearing

The issue with the dip angle changing is that the card will start tipping away from the horizontal in its gimbals. On some compasses, this can result in the card jamming and not being free to rotate in the bowl.

This happened to us as we sailed from San Francisco (with a Danforth binnacle compass purchased there) to New Zealand. When we got to ~30 degrees south, the card started hanging up at times, and before we arrived in Auckland it was locked solid. Very disconcerting it was! We took the compass to an adjuster there in Auckland and he rebalanced the card and it worked throughout the South Pacific thereafter.

It was interesting that friends with a Ritchie compass had better results. Their card dipped but never ceased to work, even when they went to Tasmania (42 degrees S).

One of the things they never tell you at the boat show!

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Old 09-06-2018, 14:06   #83
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Re: true vs magnetic bearing

Degaussing normally refers to reducing residual magnetism in ferrous metal. For reasons already stated, this is done to ships to lessen vulnerability to certain mines. A great antenna engineer and author - Dr. John Kraus worked on perfecting the process in WWII.

Generally, an AC current is applied to coils on or adjacent to the metal, and then the current is gradually reduced to zero.

https://en.wikipedia.org/wiki/Degaussing
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Old 09-06-2018, 14:08   #84
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Re: true vs magnetic bearing

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Once you get to a variation of 20 degrees +/- normal magnetic and flux gate compasses are considered quite useless and you have to rely on non-magnetic based compass systems like a sun compass of gyroscopic compass to give you a heading using the earths axis as the reference point.
I don't think that this is correct. Plenty of folks using magnetic compasses in areas with large variation. The compass doesn't care, and one just makes a larger correction in calculating one's course. Dip angle, as I describe in my previous post, is a different matter.

If you can provide a legitimate source that supports your statement, I'd be happy to read it and learn something new.

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Old 09-06-2018, 16:07   #85
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Re: true vs magnetic bearing

Quote:
Originally Posted by Wayfarer1008 View Post
I need to point out you are possibly confusing the different corrections that need to be performed on a compass. First you have to correct your compass deviation caused by metallic parts of your boat that affect the instrument itself Traditional compasses that use a magnet are the most effected and need to be corrected while Flux gate compasses generally require not deviation correction. Once corrected if you are sailing in local waters you can plot your course using magnetic north but all charts and maps are based off of true north. The correct way to set a course bering is to correct to true north by including the declination from magnetic north to true north in your course calculations. If you have a modern map platter it will automatically make that correction for you but if you use paper you will have to make that calculation. The difference in declination can be pretty significant and magnetic north is always on the move so the declination changes all the time. from place to place the changes are more significant. If you are in Miami the magnetic declination between the true north pole and magnetic is a correction of -6 degrees, 44 min. But if you are in New York and the Chesapeake the correction is -12 degrees, 48 min. Since the declination gets greater as you approach the magnetic pole by the time you reach Nova Scotia you are now at a declination of -17 degrees 39 min. Once you get to a variation of 20 degrees +/- normal magnetic and flux gate compasses are considered quite useless and you have to rely on non-magnetic based compass systems like a sun compass of gyroscopic compass to give you a heading using the earths axis as the reference point.


I believe you are incorrect on several issues.

First, fluxgate compasses are subject to deviation same as regular magnetic compass. But the compasses automatically apply the correction and the issue is invisible to the user. Fluxgate compasses need to be “calibrated” by turning the boat in circles several times at a constant rate of turn. The electronics determine what the deviation is for the boat on any heading and automatically applies the correction. Thus the effects of deviation are invisible to users most of the time and do not need to be accounted consciously. I suspect that if the vessel moves far enough that magnetic dip has changed significantly then the “calibration” would need to be redone.

Second, plotting of true headings and bearings are not more correct than magnetic. On large steel vessels working in True is more appropriate because they tend to have gyrocompasses and repeaters that display true headings so working/plotting everything in True eliminates conversion steps that can be in error.
-On vessels that primarily use chartplotters for navigation working in true also makes sense, the GPS is supplying course info in True.
-On small boats that use visual navigation and paper charts plotting working/plotting in Magentic is more appropriate because there are fewer steps in which to make math mistakes.

Finally your statement that magnetic compasses become unusable in areas with variation in excess of 20* appears to be conflating high variation with proximity to a magnetic pole. It is true the north of Nova Scotia the variation is over 20* and magnetic compasses have reliability problems. But there are plenty of places in the world that one is more likely to go that have high variation yet magnetic compasses are consistent indicators of direction. New Zealand for one, off the SE coast of Brazil and off South Africa. North of Nova Scotia one is coming closer to the magnetic pole where magnetic dip/inclination becomes significant.
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Old 09-06-2018, 17:15   #86
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Re: true vs magnetic bearing

We do this while sitting the exam for a very good reason in that if the electronics go down you still need to find your way to where you are going. If you do it enough it actually becomes second nature and takes just a few minutes to plot a course.
In the same scale all off-shore race boats that are doing deep blue races like Trans Pac and Bermuda are required to have some on board that is certified in celestial navigation.
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Old 09-06-2018, 18:03   #87
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Re: true vs magnetic bearing

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Hi together,
to answer xxhat`s question in a usable way, just rember something
we in Germany call course tower:

CC Compass course
+ DEV Deviation
= MC Magnetic course
+ DEK Declination
TC True course

If you have your compass course and want plot it in the map, just
add Deviation and Declination with the correct signand you have your
True course for the chart.

If you have TC out of the chart, just subtract DEK and DEV with the correct sign from TC and you know the Direktion to steer on the compass

Hope it helps.

John

I've never heard of a course tower (neither did google) and can't really see how that is easier to remember than the "true virgins" ? Maybe you have some links?
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Old 09-06-2018, 18:12   #88
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Re: true vs magnetic bearing

Since we are talking about north south east and west a great bit of trivia to know about it is why we always orient a map with the north to the top. In Roman through Medieval times Jerusalem Constantinople named after Emperor Constantine was considered the center of the world and all land "And Rome" radiated from their to the edge of the earth.
Egypt was the middle east, West was Crimea and the land of the Slaves and Danes, South extended to the pillars of Hercules 'now called Gibraltar" and North was the Orient and source of silk and spices. Once it was settled that the world is round and their was no center of the earth the poles took place and the point of orientation. Since the compass pointed north the compass points were moved to that point but the term Orient stayed in out language.
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Old 09-06-2018, 18:12   #89
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Re: true vs magnetic bearing

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Originally Posted by TreblePlink View Post
To summarize, the difference between magnetic and true is variation. Variation changes dramatically for different locations. In some areas where the variation is less than, say 4 degrees, for practical purposes they are much the same. However other areas can have extreme variation, so corrections are required. Charts invariably show variation for the area.

Remembering the sign for variation:

East is least, West is best.


To expand your ‘East least, West best’ jingle, I’ve always clarified to myself with “Variation East Compass is least, Variation West Compass is best”
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Old 09-06-2018, 19:21   #90
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Re: true vs magnetic bearing

I do so wish that people would stick to traditional maritime terminology..ie ...variation, deviation, and dip... and leave that other stuff to the dirt dwellers..

What next... left and right instead of port and starboard?

http://www.skysailtraining.co.uk/com..._deviation.htm

http://marinegyaan.com/what-are-erro...netic-compass/
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