Please Help: Magnetic Course Display in OpenCPN

[Viking Sailor]

Originally Posted by cagney

Dave

I think that using magnetic bearings etc comes from using parallel rulers.
Many paper charts have double compass roses one true and with a smaller magnetic inside. So it was real easy to just read the magnetic value from the inner compass rose.
Personally I just hate parallel rulers, but that's just me


I can't see that using magnetic courses and bearings etc, changes anything in OpenCPN except for the value displayed. No changes in calculations, no differently drawn vectors etc. No changes in accuracy either as long as we are talking about own ship navigation and AIS displayed from a receiver on-board. It is then reasonable to regard the variation as constant. The only thing O has to do, whenever a direction is displayed, is to instead of "true" show "true" - variation (with signs) an add an "m" to show that it's a magnetic value. It's just a question of using a different reference point for N.


There will be occasions when this will give ridiculous results. Receiving AIS over Internet from the other side of the world and using Own Ships variation, is one of them. There is nothing reasonable to do about that. It will just be a "price" the user pays if using the "magnetic" check box.


Thomas

Thomas,

The way to fix the "ridiculous results" situation is to provide a function that will return the variation for a given position. Thus, allowing the conversion of COG(T) to COG(M) as needed.

Paul

[nohal]

Paul...
The problem here is more philosophical than technical, I would say... Of course we can compute the variation at any target's position. But isn't it just adding to the overall confusion? And should we actually do anything with what the AIS target reports? Should we also recalculate the chart object info to magnetic on vector charts, for example?
But anyway, I'm just another of those guys using true, so my personal answer to all these questions simply is no...

Pavel

[JPA Cate]

Originally Posted by bdbcat

Hey there, folks...

OK, I have been working on implementing the Feature Request related to Magenetic vs True compass course display in OpenCPN.

Here is my current dilemma:

Maybe it is just me, and my training/mind set. But I am having a hard time simply converting a calculated COG, as from AIS or GPS receivers, into a magnetic course, and then showing it on the screen.

That is to say, it simply bothers me somehow to write COG(M).

I (just) might be convinced that expressing ownship COG as magnetic course is OK. Still bothers me, though, since COG, even with variation applied, has only a very loose connection with what one might read from a compass as a heading.

And converting an AIS ship reported COG to magnetic is to me non-sense.
What is the point?

None of the above applies to bearings, however. That is what one reads from a hand bearing compass. Or what you might see on a printout of a proposed route, with steering instructions. No problem there whatsoever.

So, what say you all?

Dave

Hello, Dave,

In answer to your question, I say, PLEASE go to the COG(M) if you can make it work. It would make my life so much easier, because I, too, started back in the days of piloting by magnetic bearings, using parallel rules on paper charts. In addition, I am handicapped by being possibly the world's most inept person for doing even simple arithmetic. At night, tired, I can badly mess up conversions. I need to keep it really simple. I have no idea how difficult making the change to being able to show the magnetic would be, I surely do hope it's easy. I'm only one, and most probably in the minority in my lack of ability to mentally manipulate numbers, but I will REALLY appreciate it if you do it or cause it to be done!

Thank you for considering my request.

Ann

[HappySeagull]

True North is UP on all my charts and maps so Magnetic courses/bearings are always a calculation and non-intuitive.. A very Local one at that.
Magnetic Compasses are old doofunnies,as difficult in nuance as the new doofunnies .
That said, I use one to steer straight(on the rare occasions it's possible or required)
and Pavel's handy Magnetic plugin is a big help.
PlugIns | Official OpenCPN Homepage

Maybe a course/route converter plugin could be created/added to it.

[Viking Sailor]

Originally Posted by nohal

Paul...
The problem here is more philosophical than technical, I would say... Of course we can compute the variation at any target's position. But isn't it just adding to the overall confusion? And should we actually do anything with what the AIS target reports? Should we also recalculate the chart object info to magnetic on vector charts, for example?
But anyway, I'm just another of those guys using true, so my personal answer to all these questions simply is no...

Pavel

Pavel,

True is good!

However, magnetic is the reality for most cruising sailors.

So, my philosophy is that O should make navigation for the short handed cruising crew as easy as possible. This philosophy is not supported by making them continuously convert between true and magnetic in their heads.

So, when O is set to display in magnetic, my answers to your questions are:

Yes, all own ship course information should be displayed corrected for variation and deviation (COG, routes, tracks .etc.).

Yes, all reported course information should be displayed corrected for variation (AIS, DSC, bearings .etc.).

No, vector charts should not be corrected. Since O can display both raster and vector charts concurrently -- they should be displayed consistently.

Best regards,

Paul

[nohal]

Paul...
One more question. Is this magnetic really magnetic (as in true + variation) or is it compass (as in true + variation + deviation)? Sorry if I sound ignorant, but my mind really is a bit fixed on "plot it true to make it simple" paradigm...

Pavel

[bcn]

Originally Posted by nohal

Paul...
One more question. Is this magnetic really magnetic (as in true + variation) or is it compass (as in true + variation + deviation)? Sorry if I sound ignorant, but my mind really is a bit fixed on "plot it true to make it simple" paradigm...

Pavel

In this context: in the "Properties" of a route or track we have the bearing.
If a leg is long enough the variation of this leg will be variable. How should this be handled?
AIS data: course, heading - variation at the point of the sender or the variation of the observer/own boat?

[JPA Cate]

If a leg is long enough the variation of this leg will be variable. How should this be handled?


Simply allow it to change as the magnetic variations change along the route.



AIS data: course, heading - variation at the point of the sender or the variation of the observer/own boat?

Own boat, please.

....And thank you for considering this!

Ann

[Viking Sailor]

Originally Posted by nohal

Paul...
One more question. Is this magnetic really magnetic (as in true + variation) or is it compass (as in true + variation + deviation)? Sorry if I sound ignorant, but my mind really is a bit fixed on "plot it true to make it simple" paradigm...

Pavel

Pavel,

If deviation equals zero then both expression give the same results. Since deviation is unique to a particular steering compass installation, it can only be non-zero with respect to that steering compass. Thus, it is only courses that are relative to the steering compass that need to be corrected for deviation. These would include own ship current course (COG), planned courses (routes), and past courses (tracks).

Since is it not possible to know the deviation for any other vessel's compass it stands to reason that any reported courses can only be corrected for variation.

It is also not practical to determine the deviation of a hand bearing compass and thus it can only be corrected for variation.

Note that these are just values that are displayed instead of the underlining true courses. The calculations for the above values would still be done in true and then displayed as magnetic or compass corrected values.

Paul

[bcn]

Resuming (please correct me if wrong):

- displayed bearings should be corrected by variation at ship's position (like skewing the chart by the amount of variation)

- bearings (course, heading) with relation to the steering compass should be corrected as well for deviation. This implies to have a deviation chart for that compass.

For bearings in route or track properties I don't see it. That would mean to break up the legs for every sub-leg with a different variation. Does other systems do that??

Living in the "T" world.....

[Viking Sailor]

Originally Posted by bcn

In this context: in the "Properties" of a route or track we have the bearing.
If a leg is long enough the variation of this leg will be variable. How should this be handled?
AIS data: course, heading - variation at the point of the sender or the variation of the observer/own boat?

Bcn,

The problem of a variable course of long distances is not unique. This can also occurs for great circle routes where the departure course is different from the arrival course. The usual way of dealing with this is to break the route into segments and calculate a new departure course for each segment. A similar technique could be applied to correct for the magnetic variation of long rhumb line and great circle routes.

Since magnetic variation varies slowly over the surface of the earth it makes little difference locally whether the correction value used come from the sender or the receiver. Vessel within fifty miles would be unlikely to see much difference in their magnetic variation. And, vessels greater then fifty miles are unlikely to be of much navigational concern.

However, for observers of internet AIS data it would make more sense if the correction values where from the senders. That way the interaction of local groups of vessels would tend to make more sense. But, I can't think of any reason way anyone would want to display this data in magnetic mode.

Paul

[bcn]

Originally Posted by Viking Sailor

...........
Since magnetic variation varies slowly over the surface of the earth it makes little difference locally whether the correction value used come from the sender or the receiver. Vessel within fifty miles would be unlikely to see much difference in their magnetic variation. And, vessels greater then fifty miles are unlikely to be of much navigational concern.
...........

Paul

That's why I asked - for planning purposes in Magnetic? Do you really work this way? Scratching head...

[bdbcat]

Wow...

I knew this one would not be as simple as adding T+V=M.
Thanks for all the feedback. As Pavel notes, my issue is philosophical rather than practical. COG(M) still bothers me, especially for AIS targets.

Anyway, here is my current proposal.

1. There will be a user specified variation entry field (USER_Var). It will be a constant for current instance of OCPN.
2. If user adds the WMM PlugIn, then the variation will be automatically calculated based on own-ship position. This value will override the USER_Var.
3. COG displayed on the bottom status line will be COG(M) (Much as this pains me...)
4. Bearings and Courses in the Route Properties dialog, route leg rollovers, and Active Route console will be (M), corrected for variation.
5. AIS target reports will show target COG(M). sigh....
6. OCPN knows nothing about deviation. If you have a steel boat, I guess you had better learn a little math

After I test this stuff locally, I shall never ever use it. But I hope some will find it improves their OpenCPN experience.

btw, I'm sure all realize that if you have your GPS receiver set to send Magnetic info, you will want to revert to True output, and depend on OCPN to do the math.

Ready for the next round?
Dave

[bcn]

Originally Posted by bdbcat

Wow...
Ready for the next round?
Dave

Hubert

[Viking Sailor]

Bcn,

Originally Posted by bcn

In this context: in the "Properties" of a route or track we have the bearing.
If a leg is long enough the variation of this leg will be variable. How should this be handled?
I would break the leg into segments that resulted in course changes consistent with the accuracy of my ability to steer using the compass.
AIS data: course, heading - variation at the point of the sender or the variation of the observer/own boat?
Locally - own boat. Globally - sender.

Originally Posted by bcn

Resuming (please correct me if wrong):

- displayed bearings should be corrected by variation at ship's position (like skewing the chart by the amount of variation)
Locally - own ship. Globally - source ship.

- bearings (course, heading) with relation to the steering compass should be corrected as well for deviation. This implies to have a deviation chart for that compass.
Yes.

For bearings in route or track properties I don't see it. That would mean to break up the legs for every sub-leg with a different variation. Does other systems do that??
On paper charts learned to plot noon sight and calculate (look up) new compass course. Change course as needed.

Living in the "T" world.....

Originally Posted by bcn

That's why I asked - for planning purposes in Magnetic? Do you really work this way? Scratching head...
Sorry, not sure of the question at this point. I hope that the above answers somehow address your question. If not please try again.

Best regards,

Paul