Magnetic vs True North for Electronic Nav Instrument Setting

[hardee]

This is something that has puzzled, perplexed and petrified me at times. We know there are both magnetic and true north settings available on our electronic devices. And, I think, most of us know that we can't mix those settings from device to device (VFH to Plotter or KVH to plotter, or GPS to VHF), so which one makes the most sense to use for all the devices, all the time?

Why is one better than the other, or easier to use, if it is?

It seems to me that :true" might be easier to correlate with paper IF you still use paper.

I'm curious what you are using, and why, if you care to share.

Thanks,

Harvey
SleepyC

[smittypaddler]

All my GPS devices report headings relative to true north. Typically I'll get on the desired heading using a GPS device, such as my chart plotter, then I'll check the heading on my magnetic compass, which of course is almost never is the same, given magnetic deviation and variation. Then I'll follow that magnetic compass heading, only because the magnetic compass shows more quickly if I vary off course than the GPS unit does.

[nordicstallion]

Declination is set on my compass so I'm going with magnetic, but that's just me! Vern

[nordicstallion]

Declination is set on my compass so I'm going with magnetic, but that's just me! Vern

[digger]

I guess I will stick my neck out first. I always have used the true settings for gps and for map reading or planning of routes. It has been easier for me to use the parallel rulers,etc using the true north lat/long lines. I have noted compass reading when I am going offshore so in case of instrument failure I can add the 180 to get back to visual shore landmarks.

[colobear]

Our chartplotter is set to display magnetic north, the radio is set to display true north, that allows me a quick verification of declination. I think either will work fine, just depends on what you get used to.I'm sure there is some official way to go though. And, we use course-up display on the chartplotter. I prefer that. Our backup IPad is magnetic with north up (for navionics charts at least). I get used to looking back and forth and like the options.

[journey on]

The GPS and a magnetic compass/flux gate use different methods to calculate direction or heading. Their native output is in 2 equivalent references: true for GPS and magnetic for compass.

The GPS uses geometry, based on satellite orbits to find position on a globe and then differences this position to find direction. So it's native direction/heading reading is True North.

A compass and/or flux gate uses the earths magnetic field to establish a pointing direction directly with respect to the earths magnetic field. Unlike GPS it doesn't give you a position but a heading relative to the magnetic field, or Magnetic North.

So those are 2 different heading vectors related by a fixed transformation, between true and magnetic. And of course this transformation varies with respect to your position on earth. Both are equally valid as long as you accept the difference; once you do, there is no real difference between the two.

The traditional method of using heading is magnetic since the magnetic compass was the primary measuring device. Now with GPS we have a choice of 2 equivalent headings. And electronic charts will give you either, but since they get their heading largely from GPS, one can claim that true heading is native to those charts. Either a magnetic course or true course can be plotted on a paper or electronic chart, they're equivalent.

The one to use is the one with which you're most comfortable. The only requirement is that one is consistent, you can't jump between true and magnetic without getting in trouble. So each statement as to which is the right one is true for that person. I, being brought up on paper charts and magnetic compasses complete with hand plotting, tend to use magnetic. And that choice is only correct for me.

It's sorta like twin vs single engines; both will move and steer the boat. The correct one is the choice made for a specific boat by the owner.

Boris

[hardee]

Oh great, this could be another twins / singles style. (Well I knew there would be differences) For me, with everything printed (charts) using North up, or True north, it seems easier to use that for a standard.

What brings this up is that I am running into a situation where my auto pilot is having a hard time finding which way to go, and it is apparently from having more than one directional input. This may be due to different "rates of output" from more than one GPS and direction sensor (flux gate compass) getting a signal into the AP somehow (possibly via the MFD.)

So for now, I'm thinking I want to be sure everything is all looking at the same "north", and for me, if they will, they should be all saying the same signal, "North is True".

Boris, thanks for the excellent explanation. You are right, they both will work --> "

[hardee]

Sorry -- Duplicate post.

Harvey
SleepyC

[Aurelia]

[colobear]

Harv,

I like the true north on the radio and magnetic on the plotter so when my chart indicates an area of local magnetic disturbance I can glance up to see any effect, nothing critical, other than that No real reason, just what I am used to.

[thataway]

When we used to use the compass for navigation, we always computed course relative to magnetic bearing or course. We plotted the true course and then corrected for declination and variation. Today we rarely use the compass--the exception is taking a bearing on an object, or the course out of a harbor should we have to leave suddenly, before the boat begins to move--that we have the proper course. (Now the latest GPS have a update rate of 10 Hz--that is the GPS heading and position is recalculated 10 times a second, rather than once a second, and is far more accurate. )

The declination changes at any one place at varying rates--and even if you are always in the same place, you need to re-calculate the variation. It would seem to be difficult to always to rotate the compass as you travel or come into new areas. The deviation (due to effects of magnetic fields on the boat), also changes, as you rotate the compass, and a new diagram has to be made or the compass re-calibrated. I certainly would always want the
the compass to be reading compensated magnetic, or at least have a deviation diagram available for the compass so you can tell what the actual magnetic heading is. Generally the C Dory compass is fairly easy to compensate--unless you start putting electronics around it, wires which carry current some times, and speakers nearby--as many of us do…..

Even for those of you who boat in the PNW, as you go up the inland passage the declination changes from Seattle today: 16° 12' 20" E changing by 10.8' W per year

to Juneau AK: 20° 14' 26" E changing by 19.3' W per year.

A very handy web site is at: http://www.ngdc.noaa.gov/geomag/geomag.shtml You can find your declination by clicking on "Declination" on the first set of links--and this takes you to a page where can enter your Zip Code, City, or Lat and Log.

http://www.ngdc.noaa.gov/geomag-web/#declination

Also I use a sun compass and tables to check my compass--and that is referenced to magnetic readings.

With the GPS, for me it just more intuitive to use true. I do like to run with course up display, that way I can see on the chart, what I see relative to my radar and visual fields.

[journey on]

Well, Bob, I certainly agree with what you do, though I'm not as careful. You knew that, huh? However, as we cruise along on Journey On, I have plenty of time to think about how we're steering the boat, and I've come to a couple of conclusions. One conclusion is that the compass course isn't that important to me, but arriving at the correct destination is. I select a planned route, point Journey On in the direction the electronic chart plotter/GPS tells me and then steer by the crosstrack error. Any inaccuracies in Journey On's course are automatically corrected.

Journey On has 3 course indicators on board; 2 magnetic and one GPS. The magnetic indicators are a visual steering compass and the autopilot fluxgate. And of course the GPS is the GPS.

Normally when we're cruising, I set a waypoint, or even a whole route, then point the boat to where the GPS display/plotter tells me by setting that direction on the autopilot. Then I steer not by any direction but by the cross track error and adjust the autopilot setting accordingly. Since the autopilot uses magnetic, that's where I set the GPS. Yes, it would be easier to just set the autopilot to track the desired course by using data directly from the GPS, but that doesn't work as easily as it sounds. And I really don't care what reading the autopilot is giving, since I'm using cross track error which is neither magnetic or true. However, I do need consistency/stability in the reading.

Another mode used, for example, is when we come back from Santa Cruz Island around noon and the downwind swells are more than the autopilot can handle. I manually steer and the GPS indication isn't fast enough, so I use the magnetic compass to try to hold a reasonably straight course. I update my course again by using the cross track error. Again, compass accuracy isn't important, which is good because Journey On is usually bouncing all over the place. Again, consistency/stability in the compass reading is important and the magnetic devices have this.

So you can see that the steering criteria is to minimize the cross track error and drive it to 0 when we arrive at our destination, not before. In order to minimize confusion and panic, I try to make sure all devices read in the same reference frame: magnetic. And there is no need to calibrate either magnetic device, because using cross track error automatically compensates for any deviation between the 3 devices, as long as the GPS doesn't screw up.

This change in the way we used to navigate and the new way has amazed me. Years ago, I would set a compass course to Avalon, allow for the southward current and wait until Avalon appeared and then steer to the breakwater. Now I just set the waypoint navigation and adjust the autopilot heading and wait until we're there, completely different. Yes, this means I need an operating, active GPS and autopilot. I do have paper charts on board, but neither the plotter, GPS or autopilot has quit in mid-flight yet. And I sit there and think that the steering compass isn't the essential instrument it used to be.

Even if I'm doing manual piloting (gasp,) I point the boat on an approximate desired course and update it by visual aids, certainly more accurate than dead reckoning.

Oh, and now we have radar to see if anything big is approaching.

Time have changed. And I enjoy thinking about how they've changed. So regard this post as just a bunch of freebe comments.

Boris

[thataway]

Hi Boris,
Interesting thoughts as always!--and when I had boats with auto pilots and GPS, I did steer by cross track error. I have not had a auto pilot on any of the C Dorys. Why no auto pilot? Down on the Gulf, I generally am going along the ICW, or toward a fishing location--with a fair amount of boating traffic, thus I am hand steering. Powell, Central Calif, So. Calif and PNW--pretty much the same way. I misspent a lot of my youth watching a compass in a slow sailboat. I guess that the psychology of watching the cross track error when hand steering not appeal to me. But I find a lot of my "navigation" is watching the water, other traffic, and an occasional glance to see where we are in relation to the course line on the GPS. I do run two chart plotters--one is set for the scale of that segment of the route. The other is set on a much more magnified scale--and I glance back and forth between them.

[hardee]