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Old 31-01-2013, 13:31   #46
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Re: True or Magnetic for Electronic Nav

Hi Jim,
If you are doing all your navigating with a standard magnetic compass, then there is no “shame” only admiration.

But the OP’s question related to harmonizing with modern electronics driven by GPS

“Correcting” is converting a magnetic (M) direction to true (T).
“Un-correcting” is converting from true to a magnetic direction.
“True” direction is relative to true north (north pole).

This was the terminology used in Marine colleges and text books… happened to find it also used in this link
TVMDC

I think we actually probably do the same thing, except my electronics default to True North

This is how part of my watch log is set up… (set heading is the auto-pilot course from flux-gate)

TRUE COURSE // STEERING COURSE M // SET HEADING


By the way….. Gyro headings need to be ‘corrected’ also… both for high or low calibrations and latitude corrections
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Old 01-02-2013, 01:25   #47
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Re: True or Magnetic for Electronic Nav

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Originally Posted by Jammer Six View Post
I've never seen a reason to use true. I still don't.
Now you can see how a man gets confused.

I read a bunch of posts from guys whom seem like experienced seaman whom say use true, and state their reasons.
And then you come in and just say straight to the point that you see no reason to use true and never have!

Extrapolate if you may, but no pressure.

Cheers
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Old 01-02-2013, 01:41   #48
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Re: True or Magnetic for Electronic Nav

FWIW, aviation gyro compass' are referenced to magnetic not true. Ground aids and runways are referenced to magnetic.

GPS sets can be programmed to either but normally it is set to magnetic. True doesn't get a look in.

At least in Oz, just sorta assumed in would be the same elsewhere.

I use magnetic on-board but passage plan in true and then convert as required.
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Old 01-02-2013, 02:14   #49
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Re: True or Magnetic for Electronic Nav

Is not this all rather silly? Hairsplitting? All the charts are in True as there is no such thing as a Magnetic coordinate system. There are no Magnetic latitudes, longitudes, distances, speeds ... only Heading and Heading's advisory cousin Course to Steer. Even Bearings are problematic in Magnetic, for the picky.

If you want to steer by the magnetic compass then have the chart plotter give Magnetic Course to Steer or noodle it in your head.

WX in Magnetic? Ya gotta be kidding?
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Old 01-02-2013, 02:33   #50
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Re: True or Magnetic for Electronic Nav

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Originally Posted by daddle View Post
Is not this all rather silly? Hairsplitting? All the charts are in True as there is no such thing as a Magnetic coordinate system. There are no Magnetic latitudes, longitudes, distances, speeds ... only Heading and Heading's advisory cousin Course to Steer. Even Bearings are problematic in Magnetic, for the picky.

If you want to steer by the magnetic compass then have the chart plotter give Magnetic Course to Steer or noodle it in your head.

WX in Magnetic? Ya gotta be kidding?
Apparently not to the previous 48 posters!

But to be even more picky, charts are usually (always?) in both; co-ordinates (lat / lon etc) are in neither as the concept of true and magnetic only apply to bearings and azimuths. Neither does the concept apply to speed or distance.

I defy you to be able take a "true" bearing on the average sailboat using the traditional hand bearing compass, I certainly can't; well I can when I use my adjustable hand bearing compass but I have never seen anyone using using a adjustable hand bearing compass (but I am sure someone else on CF has one )

And while not exactly on topic, as previously posted, all airport runways are referenced to magnetic so it is not just some dark distant unused concept in the general world of navigation.

The question remains valid, which option on your electronic nav stuff do you use - true or magnetic. Neither is holier than than the other; both are valid and both have pros and cons.

YMMV
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Old 01-02-2013, 03:30   #51
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Latitude and longitude are absolutely defined in terms of True directions. As are all charts. As therefore speed is as well, being derived directly from angle.

I use Magnetic to steer by, and so does my autopilot, because we prefer to steer by magnetic compass. And bearings are read from a magnetic compass. But I am we'll aware that beyond that lies only nonsense and insanity.
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Old 01-02-2013, 03:31   #52
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Re: True or Magnetic for Electronic Nav

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Originally Posted by Wotname View Post
FWIW, aviation gyro compass' are referenced to magnetic not true. Ground aids and runways are referenced to magnetic.

GPS sets can be programmed to either but normally it is set to magnetic. True doesn't get a look in.

At least in Oz, just sorta assumed in would be the same elsewhere.

I use magnetic on-board but passage plan in true and then convert as required.
The first aviation gyro compass were directional gyros. They did not seek north like a maritime gyrocompass does. The compass was self leveling, but would precess. It was actually necessary to periodically align them, you guessed it to the magnetic compass. When aviation was in its infancy, magnetic compasses were the only heading reference. When equipment advances were made the magnetic system was already well in place.

A self aligning compass becomes inaccurate near the poles. This should not affect most of us for a few more years of global warning!

As a side note the Canadian Northern Domestic Airspace uses true tracks.


Back to the OP - All of my planning is done in True, the chart plotter at the nav station is always in true and zulu time. All logs and fixes are in true. My computer programmes are in true, as is any spreadsheet that I use. At the helm, the admiral prefers magnetic! Easy enough to apply variation and deviation as needed.
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Old 01-02-2013, 04:51   #53
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Re: True or Magnetic for Electronic Nav

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Originally Posted by daddle View Post
Latitude and longitude are absolutely defined in terms of True directions. As are all charts. As therefore speed is as well, being derived directly from angle.

I use Magnetic to steer by, and so does my autopilot, because we prefer to steer by magnetic compass. And bearings are read from a magnetic compass. But I am we'll aware that beyond that lies only nonsense and insanity.
Nit picking here, but speed is a function of distance and time. It is the magnitude of your velocity. Your velocity will have a direction.
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Old 01-02-2013, 21:49   #54
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Re: True or Magnetic for Electronic Nav

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Nit picking here, but speed is a function of distance and time. It is the magnitude of your velocity. Your velocity will have a direction.
Yes. But what I meant, and did not express well, is that speed in a boat is an angular velocity, as a nautical mile is equivalent to minute or arc. In the Magnetic coordinate system (see Bowditch) such a value is unworkable.

This mostly applies to the idea of a chart plotter "being in Magnetic mode", which from a software point-of-view is well beyond easy definition or implementation.
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Old 01-02-2013, 22:47   #55
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Re: True or Magnetic for Electronic Nav

Let's see.... I went on board a delivery boat as navigator. New boat that the owner claimed that the compass was compensated and has a table to go with it. Funny thing was that we kept heading for land every I sent a course to the helmsman. Not b y chance I had my trusty hand bearing compass. The ships compass was off by 45 degrees

I do navigation by true. There are magnetic poles and magnetic grid lines. However they are not straight.

With electronics I set in a true course and get us on it. With a low divergence and XTE I read the magnetic compass. That is the course to steer. All corrections applied.

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Old 02-02-2013, 01:22   #56
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Re: True or Magnetic for Electronic Nav

Quote:
Originally Posted by daddle View Post
Yes. But what I meant, and did not express well, is that speed in a boat is an angular velocity, as a nautical mile is equivalent to minute or arc. In the Magnetic coordinate system (see Bowditch) such a value is unworkable.

This mostly applies to the idea of a chart plotter "being in Magnetic mode", which from a software point-of-view is well beyond easy definition or implementation.
Daddle, you must be a lot smarter than me 'cause I can't understand your reasoning at all. Please help me out and explain.

As I understand, velocity is distance over time in a constant direction. It matters not a toss how the direction is defined providing it is constant. Speed is simply distance over time. In boatworld we usually use knots as the units of speed. A knot is simply one minute of latitude (nm) in one hour. Lines of Latitude is not defined in any way by reference to magnetic north or true north. It is pure coincidence that true north is taken as the "top" of the earth. If true north was defined in any other way, latitude would remain the same as it now is.
Thus speed is not dependant on the north reference whether that be true north, chart north, magnetic north or any other north.

As for a chart plotter (or any other electronic nav system), going from a true north to a magnetic north is quite simply a push of a button. Providing the database is current, the unit knows the magnetic variation at its current position and also that at any nominated waypoint and thus applies the variation and plots accordingly. Its not rocket science - at least I don't believe it is but as I am not a rocket scientist I could be mistaken.
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Old 02-02-2013, 01:56   #57
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Re: True or Magnetic for Electronic Nav

A further observation, assuming you sail with paper charts and electronic aids " essential for any offshore trip" running all your electronic aids in magnetic is the best practice, this allows you to have one approach to navigation where course to steer, bearing to waypoint etc can be relay to the helmsman from compass, plotter or ipad without need for any mag var adjustment. Using one of the rotaplotter tools where you can dial in the mag var and place these onto the chart makes cross checking fixes taken from hand bearing compass and distance to waypoint from GPS a simple task. For my mind the chart is the only place true is displayed and any heading data from electronic aids can be actioned by the helm on the steering compass with ease.
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Old 02-02-2013, 03:31   #58
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Re: True or Magnetic for Electronic Nav

in my opinion all plotting should be in True and only when steering orders are been issues are conversions done into magnetic. The only reason we have magnetic on board is we still rely on a traditional compass ships and others using gyroscopes have long since moved to True. Magnetic directions should remain in the realm of steering and nothing more.
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Old 02-02-2013, 04:10   #59
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Re: True or Magnetic for Electronic Nav

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Originally Posted by Jammer Six View Post
I've never seen a reason to use true. I still don't.
I don't quite get this, how does that work? Charts are drawn with true north, do you convert to mag every time?
Seems a lot of work, tidal vectors are true, light ranges are true, why not just convert for deviation and variation at the end?
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Old 02-02-2013, 04:48   #60
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Re: True or Magnetic for Electronic Nav

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Daddle, you must be a lot smarter than me 'cause I can't understand your reasoning at all.
Not if I cannot clearly explain it

It is obvious that when using True headings, bearings and such that one is using the well known and simple coordinate system of latitudes, and longitudes. All the numbers we use work together nicely. Direction, location, speed, and time (and velocity) all relate in simple ways. The coordinate system is all nicely regular, has nice (spherical) right angles, and so forth. Computations in your head, on charts, and in the chart plotter software are simple and well known.

What it less obvious is that the Magnetic headings, bearings and such that we use are also in a coordinate system. It's a crazy one. The latitudes and longitudes are not regular, straight, or consistent in any way. They even change with time! Computations in a Magnetic coordinate system would lead to insanity. Smart guy Bowditch (the book anyway) will back me up on this.

So when one sets their chart plotter to Magnetic all they are really doing is changing a few of the outputs based on the local magnetic variation. Don't fool yourself that you are working in Magnetic. Personally I think trying to mix the two is a bit nutty. I convert from one to the other at the companionway stairs when climbing up with a course to steer or climbing down with a bearing to an object. That is simple - especially so in SE Asia where the variation is nearly zero everywhere.
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