Please Help: Magnetic Course Display in OpenCPN

[Tingum]

I just love running boats with gyro compasses, makes all this discussion unnecessary.

[Stargazer1942]

Originally Posted by Viking Sailor

Dave,

I agree with Jim that being able to navigate using magnetic values is a very useful feature.

If one should find them self doing a coastal passage with a electronic chart that is not using the WGS84 datum, it would still be possible to use the chart. By using magnetic bearings and depths it would be possible drop waypoints showing the boat's track relative to the shoreline. And, of course, it would be helpful in determining the compass course to be steered when one doesn't have a plotter at the helm.

BTW, I just upgraded to 3.2.2 and was very disappointed to discover that the range ring feature has been removed. I don't really understand why a common feature like that would ever be removed. Please put it back in.

Thanks,

Paul

As the originator of this thread, I can give you a concrete example of where magnetic bearings were essential. A few years ago friends of mine were doing the Newport Bermuda Race in the Cruising Division. They had taken on fouled fuel and in a short period time they were unable to recharge their batteries. They had to turn off all non essential electronics, including their instrumentation. They had a laptop with a usb gps and charting software, as a backup, but they could only run it for short periods of time to preserve its battery. They were reduced to steering by binnacle compass. Not every member of the crew was an accomplished navigator. True bearings, would not have been as helpful nor safe as magnetic bearings.

Norm

[nohal]

Originally Posted by Viking Sailor

BTW, I just upgraded to 3.2.2 and was very disappointed to discover that the range ring feature has been removed. I don't really understand why a common feature like that would ever be removed. Please put it back in.

Thanks,

Paul

Paul...
BTW, this feature was not removed.

Pavel

[Viking Sailor]

Hi Pavel,

Ok! Where is it? I did RTFM, and looked every place I could think of to try and find it. No joy! Please make me look like a fool and tell me where it is.

Thanks,

Paul

[Viking Sailor]

Hi Pavel,

Never mind -- I am perfectly capable of looking like a fool on my own -- thank you. I did find it. Sorry for all the bother.

Paul

[El Pinguino]

Originally Posted by Stargazer1942

As the originator of this thread, I can give you a concrete example of where magnetic bearings were essential. A few years ago friends of mine were doing the Newport Bermuda Race in the Cruising Division. They had taken on fouled fuel and in a short period time they were unable to recharge their batteries. They had to turn off all non essential electronics, including their instrumentation. They had a laptop with a usb gps and charting software, as a backup, but they could only run it for short periods of time to preserve its battery. They were reduced to steering by binnacle compass. Not every member of the crew was an accomplished navigator. True bearings, would not have been as helpful nor safe as magnetic bearings.

Norm

Surely they would have had... at the very least ....a paper chart of the North Atlantic which would have had isogonic lines on it? From there its not so difficult as the magnetic variation in the entire North Atlantic is 'west'. 'Variation East- Compass Least, Variation West -Compass Best ' so you just add the variation to the true course.

If the feature is added to OpenCPN I reckon that True or Magnetic should be displayed in the same manner that Metres/Fathoms/feet is currently shown.
I was once given a hell of a fright when some bright spark decided to change the depth sounder display from metres to feet without telling anyone.

[sy_gilana]

FWIW...

I do everything in true, but make myself aware, on a per watch basis what the variation is, deviation is constant, and have, in my mind the mag bearing. Especially effective at night, as an exit bearing from an anchorage, I can haul anchor and get underway on a mag bearing if the poop hits the impeller. Only used it once, in North Bimini, but then anywhere west would have been ok. The other time I use it, much more regularly, is in electrical storms, we go dark ship, everything off, all electronics etc. and steer by hand with the compass.

Point is, I am aware of the binnacle compass bearing when I am navigating to a true course. Its not even arithmetic, I look at the compass, and say AHA TVMDC !

IF its going to be implemented by popular demand, personally I would not like to see it permanently displayed. Please make it like a UNIT preference, meters or fathoms, etc. I would not like to be confused by a mag brng while doing chartwork which is always done in true... It should be easy to apply the Var in any given area, and even have a Dev table for the user to be able to input the compass deviation corrections for his vessel, then its just + and -.....

just sayin'

[El Pinguino]

Originally Posted by sy_gilana

TVMDC !

True Virgins Make Dull Companions

[Amnesia II]

Originally Posted by El Pinguino

True Virgins Make Dull Companions

Perhaps I'm a bit slow here but I can't make EASY sense o' that one!

I quess it's what you've always used that's easiest. "Variation east compass is least, variation west compass is best".

Explained for newbies:
Variation East compass is least....... your magnetic or compass course to steer by is the chart course(true) MINUS the variation east value or PLUS the variation west for your area.

e.g. Chart course 345 degrees - 3 East var = 342 magnetic (compass)
Chart course 058 degrees + 7 West var= 065 magnetic (compass)

[El Pinguino]

Originally Posted by Amnesia II

Perhaps I'm a bit slow here but I can't make EASY sense o' that one!

No need to really, its primarily designed for steel ships where, especially if the variation is large , it is considered important to apply variation before deviation. As you say all a small boat sailor really needs to know is 'error east, compass least'.
Getting back to the OP's comment 'Not every member of the crew was an accomplished navigator ' in a case like that the skipper should just be giving the watchkeeper a magnetic course to steer thereby removing the need for thinking on the part of said watchkeeper.

[mileskb]

I might as well pile on... Every line I draw on the chart, and all my gear on the boat are in MAGNETIC. If the CG asks for a bearing to somewhere or from somewhere or someone is located xx degrees off the tip of someplace... it's magnetic. True if for paper charts. Now I realize different strokes for different folks, but for me.. why should I "convert" every single turn from true to magnetic.

My routine has been to plot my course in OpenCPN, then export the route and import the route into Garmin Homeport which is set to Magnetic so I can print the route out (waypoints and turn info) in something that is simple to use looking at a compass.

I know this isn't a solution for everyone. My deviation is generally 17 degrees for the area I boat, so just leaving the Charplotter in Magnetic mode makes it and the Compass line up just fine.

Finally the LAST thing ANYONE wants to do is put their life in MY hands based on my math skills in a tight situation.

Thanks for a FANTASTIC piece of software. It would be the ONLY piece of software I needed if I could plan my courses in Magnetic and not have to import into something else to convert.

[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

[Viking Sailor]

Dave,

For COG(M) is always COG(T) corrected for variation. This is what should be plotted/displayed on the chart.

The Compass Course (COG(C)?) is the COG(M) corrected for deviation. Since O can only know its own ship deviation it can only calculate its own ship Compass Course. This implies that a deviation table or curve is a available to O. A deviation table (N, NE, E, SE, S, SW, W, NW) could be entered or even learned. And, once a table is entered or learned a curve could be fitted to the table to aid in interpolating the compass deviation. I believe that this is what should be reported on the status bar in place of COG(T).

The ships heading (what the compass actually reads) is the Compass Course altered by the environment (i.e. currents, wind, sea state, .ect.). This can result in a difference between what the COG is and what the compass reads. However, this is true for either magnetic and gyro compasses.

Paul

[Jim 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

G'Day Dave,

I am not sure that I understand your question. Are you saying that there is a logic problem in generating the COGM data, in displaying it on OCPN, or that you don't like doing it for some personal reason ("just seems wrong")?

In my world, I set the display on my GPS to read out in magnetic... COG, CTW and so on. The GPS then tells me what compass heading I should steer to arrive at my destination. My autopilot, when in use, is set to display magnetic headings... everything is done in magnetic. I'm not defending this practice, just describing what I have successfully done for years, what is possible in other charting software (Maxsea for instance), and what I would like to see in OCPN.

I hope that this makes some sense to you. I appreciate what you are doing for us, and hope that a satisfactory solution is reached.

Cheers,

Jim

[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