Compass Deviation Plugin future requests.

[rooiedirk]

While working on a new Compas Deviation plugin, I could use some fresh ideas.
What there is now:

1. Possible to use multiple compasses on multiple ships
2. Make your one deviation table by taking compass bearings. (Als sun's azimuth possible)
3. TODO but on my list is resending a nmea hdg message including true north course.
4. Print the deviation table.

This is how it looks so far.
Dirk

[rgleason]

Rooie Dirk. Nice idea.

Is this going to help with figuring out the deviation, or is it just for making the corrections?

Will the compass indications include deviation with the assumption we are using a regular compass rather than say gps compass.

[rooiedirk]

The plan is both. For a regular compass it will help to make a deviation table on paper. (in theory taking as less as 5 bearings should be enough)
And for an electronic (nmea) compass it can, in cooperation with the variation PI, correct the magnetic course up to a true course.
Mind you, an electronic compass with auto compensation doesn't mean that the deviation is zero. It has, just as an ordinary compas has, a remaining deviation after compensation.

[Rasbats]

Need to be a little careful here or one could get a false sense of accuracy. Only 5 compass errors will not allow a deviation table to be made. We need the vessel to be heading on all the cardinal points, North, South, East and West. And also the intercardinals, NE, SE, SW, NW, as a minimum, taking the deviation on each heading.

Recording the deviation on courses within about 5 degrees of each of those headings will not affect the result too much. So perhaps build up the information but not allow the table to be made until the minimum number of readings have been taken.

A useful addition could be the production of a table of coefficients. If you see an 'A' coefficient this would indicate a lubber line not aligned with the fore and aft line of the boat.

Mike

[rooiedirk]

Originally Posted by Rasbats

Need to be a little careful here or one could get a false sense of accuracy. Only 5 compass errors will not allow a deviation table to be made. We need the vessel to be heading on all the cardinal points, North, South, East and West. And also the intercardinals, NE, SE, SW, NW, as a minimum, taking the deviation on each heading.

Recording the deviation on courses within about 5 degrees of each of those headings will not affect the result too much. So perhaps build up the information but not allow the table to be made until the minimum number of readings have been taken.

A useful addition could be the production of a table of coefficients. If you see an 'A' coefficient this would indicate a lubber line not aligned with the fore and aft line of the boat.

Mike

I do fully agree. 5 is the theoretic mathematic minimum., but for praxis the more the better and at least in every quadrant. I do show the coefficients. (see last picture)
I can imagine it is a good idea to show additional curves for each bearing with an offset of +-1 degree error. That should give a nice idea of the accuracy of the result.

[rgleason]

Thank you. Looking forward to this. Will check mine which was swung sometime ago.
Just learned that fluxgate's have deviation's too. I am sure there will be many interested in testing, and I am one of them.

[rgleason]

Mike, foregive me, what is a table of coefficients and "A" is there a sourcelink to read and study this? I like you lubber line check idea as good supplimental feature.

[Rasbats]

Dirk,

Apologies, I missed your coefficients. Not sure about the text alongside each!

Rick,

Cape Compass:Compass Deviation Analysis Explained

And far more detailed here:

http://msi.nga.mil/MSISiteContent/StaticFiles/HoMCA.pdf

Mike

[rgleason]

Thanks Mike and Dirk. I think I'll make a new supplimental page about this which we can use with the plugin. Very helpful.

[NahanniV]

Originally Posted by rooiedirk

While working on a new Compas Deviation plugin, I could use some fresh ideas.
What there is now:

1. Possible to use multiple compasses on multiple ships
2. Make your one deviation table by taking compass bearings. (Als sun's azimuth possible)
3. TODO but on my list is resending a nmea hdg message including true north course.
4. Print the deviation table.

This is how it looks so far.
Dirk

What about an automatic mode. Recording bearings while turning the ship at a constant rate ?

[seandepagnier]

Multiple incorrect compasses??

Why not correct the compass so that it is correct?

[OZwingle]

I have long wanted a compass deviation plugin. Since OpenCPN 'knows' True and Variation (from WMM plugin), with Deviation we can have the Compass course to steer.

I would be happy to share a draft 'functional specifications' document that I have been working on, to compare with work-in-progress.

And I would be very happy to do some alpha and beta testing.

Thanks for working on this. I look forward to helping out.

[rooiedirk]

Summer is almost over, so time to pickup again.

Multiple incorrect compasses??
Why not correct the compass so that it is correct?

Multiple compasses, as I know several people that use OpenCPN on a laptop on different ships.
And you should still correct your compass offcourse. And for us on aluminium boats the deviation will be (almost) zero. But on a steel boat even a compensated compass may have a deviation of a 4-5 degrees.
(I also do compensate compasses, and if I don't get the resulting deviation below 5 degrees it is free of charge)

I would be happy to share a draft 'functional specifications' document that I have been working on, to compare with work-in-progress.

Yes please. Always good to share thoughts.

[rgleason]

Sean always gets to the nub of the problem. If the fluxgate needs correction too, what will happen? Maybe the cell phone compass would help? 3 compasses = better chance of correction? But how to get the cell phone data?

[rooiedirk]

A cellphone will be of no help. Compass deviation is caused by the vessels own magnetism.
By compensating you try to add a few magnets that will be just as strong but in opposite direction then the ships magnetism. The ships magnetism influence will be different for each place onboard. This means if you have a movable compass you cant compensate it.
What I try to do is allow to take bearings to a known point (is known bearing), and so find the misreading of the compass. The misreading is deviation + variation.
For a known bearing you could choose from a leading line(lights), a fixed point or a bearing of the sun.