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Old 07-02-2013, 07:10   #106
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Re: True or Magnetic for Electronic Nav

That's interesting ... never realized that an over time magnetic variation change is not part of an algorithm. It is clearly indicated on all nav charts and topo maps. Strange! Lazy!
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Old 07-02-2013, 07:37   #107
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Re: True or Magnetic for Electronic Nav

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That's interesting ... never realized that an over time magnetic variation change is not part of an algorithm. It is clearly indicated on all nav charts and topo maps. Strange! Lazy!
My thoughts also!

I can mostly defend the design in the days of magnetic variation in a GPS with low/no cpu power. But in today's chartplotters it should not be a problem to do it correctly.
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Old 07-02-2013, 07:40   #108
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Re: True or Magnetic for Electronic Nav

The vast majority of modern Nav systems apply the rate of change each year to the data. Equally most display the variation and allow you to input the more appropriate one. So the point is moot.
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Old 07-02-2013, 08:40   #109
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Re: True or Magnetic for Electronic Nav

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The vast majority of modern Nav systems apply the rate of change each year to the data. Equally most display the variation and allow you to input the more appropriate one. So the point is moot.
Gee Dave, that hurts! I still considered my Furuno NavNet3D as modern!

Apparently, IYO, it's vintage gear as neither the MFD (latest software) nor 330 GPS (latest firmware) adjust magnetic variation with time. Nor does the MFD have any UI where one can muck with magnetic variation.

As of 1 hour ago -
The MFD reports on the N2k: 5d 6m (2 year old software)
The GPS reports on the N2k: 4d 48m (5 years old)
Actual per NOAA: 5d 17m (change is 6m/yr)

I'll check the Simrad NSO next chance I get, it was spot on 2 years ago at the time of a software update.
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Old 07-02-2013, 14:13   #110
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Re: True or Magnetic for Electronic Nav

You can't calculate a Great Circle route in Magnetic. You can't do a Rhumb Line in Magnetic. You can't even check your magnetic compass without referring to True North. And even if you simply assume that your compass is correct because it once was approximately sort of in the ballpark, and don't create your own GC or RL track, but instead rely upon a magic box to hold your hand and do it for you, you still have to correct for Deviation, anyway, which is individual for a particular boat and a particular magnetic compass installation and even a particular heading, since Deviation changes with heading. You will probably get where you are going without referring to True North. Sort of like you will probably get to Europe if you sail toward the rising sun and then turn Right or Left when you get to land. Once again, I refer you to the most widely acknowledged authority on navigation, American Practical Navigator, AKA "Bowditch". If you are not familiar with this essential book, I will categorically state, and beg anyone to differ with me at the risk of being laughed at by real Navigators both recreational and professional, then you are NOT a Navigator. A real Navigator uses electronics as a valuable, every day, time-saving, practical tool, but does not NEED them, is not helpless when they fail for whatever reason, and can still navigate without them.

You don't need to be a Navigator to sail in coastal or inland waters. But it would be very reassuring for there to be a Navigator onboard when making a long ocean passage and a prudent skipper would agree.

BTW, Bowditch is available as a free PDF download for those who don't think it is worth spending money for, or who simply don't have room on their boat for another big book.
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Old 11-02-2013, 23:14   #111
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FWIW,

I prefer to work in magnetic like many others who rely upon magnetic compasses. In this part of the Pacific the variation ranges between around 10 to more than 15 degrees, and we move around enough that it changes significantly from week to week.

Before I changed from Max Sea to OPCN I was happy to set the parameters to read out in magnetic and all was well. So far with OPCN this has not been an option, despite periodic requests for this feature to be added. Of course Ann and I have managed to continue navigating, but it is an irritation and an occasional source of error for us.

We continue to hope that the good folks who put so much effort into maintaining OPCN will someday hear our plea and add this minor feature (HINT, HINT).

Cheers,

Jim
Jim, why did you change from Max Sea to opcn?
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Old 12-02-2013, 03:09   #112
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Re: True or Magnetic for Electronic Nav

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Jim, why did you change from Max Sea to opcn?
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G'Day Bazzer,

Several reasons...

Most important, the version of Max Sea that I inherited with a computer was fairly unstable and crashed frequently.

Second, it didn't have a friendly relationship with CM93 charts.

Third, it was an outdated version and I didn't want to be paying for updates all the time.

Fourth, because of the big stack of bells and whistles that I didn't use it was unnecessarily complex and bulky. Later versions were even worse in this respect.

Finally, the support offered for OCPN is simply outstanding, and the continuous improvement is heartening. I think that the development team is superb and I want to support their efforts.

No regrets whatsoever in making the change.

Cheers,

Jim
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Old 12-02-2013, 05:22   #113
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Re: True or Magnetic for Electronic Nav

OCPN rocks. I wish it was ported to Android.
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Old 12-02-2013, 08:49   #114
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Re: True or Magnetic for Electronic Nav

(sarcastic but nice mode on) Human error plays such a small part in sailing and in most things we do. Just ask the Captains of the Titanic, The Andrea Doria and the Costa Concordia.... (Sarcastic mode off)

They had batteries. Something like 400 pounds of batteries.

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You see - this is human error. This is their failing not some unforseen act of god that caused their electronics to fail. Excellent looooong lasting AA & AAA batteries like eneloops and the like, are widely available. You should never ever run out.

You said they did not have solar power. Again - that is human error. I would say you are nigh on close to being an idiot if you are off the grid and only reliant upon an engine to power your batteries onboard.

If you have a casual approach to powering your various electronic systems, you would probably have been as likely to be just as casual in the pre-electronic age in having all the charts, up to date almanac, sight reduction tables etc etc. Heck - I bet their were even people whom didn't have or lost their last pen and pencil when all coastal and offshore navigation was calculated onto paper. That's not the fault of the nav method its the fault of the person. The same with electronics.
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Old 14-04-2013, 15:44   #115
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Re: True or Magnetic for Electronic Nav

The unpleasant fact is that you will have to use both True and Magnetic. Your GPS will compute everything in True, your autopilot compass will will compute in Magnetic. The charts on your plotter will be True North Up. The radar display will normally be referenced to the boat heading from the magnetic compass.

Yes, you can have the electronic brains do the conversion between True and Magnetic, but to be safe, you need to understand where the electronic variation data comes from (GPS, MFD, computer, etc.) and how accurate the variation data is (one of the more commonly used model is only good for 5 years) and how the devices apply the data.

So, IMHO I let the devices work in their "natural" mode: AP reads and steers magnetic, GPS reads and computes in True. That way I at least know who made the conversion mistakes.
Rick
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