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Old 31-01-2016, 14:07   #1
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Variation and deviation, online Nav course, please help.

Hello everyone.

I seem to have greatly confused myself. I am working on nautic ed's online navigation course, and the section that is trying to explain variation and deviation has me all flipped around.

The course is trying to get you to use a graphs and pnumontics, to remember how to do this, however I find this much more confusing then just learning how it works. I have a back grounding engineer, and surveying, so angles are second nature to me, but in this work I find I have trouble figure out when to use plus signs, and negative , and what the equations are. Further, I seem to become greatly confused when they say you have to "subtract "E", or something like that because to the east would be negative already. Further confusion exists because sometimes I feel like it is talking not about positive or negative but magnitude. So if for example I have deviation of - 2 degree west, and it ""decreases by 30 mins"" does that make it 2-30 or 1-30?

Can anyone either explain this to me, .. or point me to a reliable website that might make this clear?


Kevin.
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Old 31-01-2016, 14:21   #2
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Re: Variation and deviation, online Nav course, please help.

Quote:
Originally Posted by kevinnem View Post
Hello everyone.

I seem to have greatly confused myself. I am working on nautic ed's online navigation course, and the section that is trying to explain variation and deviation has me all flipped around.

The course is trying to get you to use a graphs and pnumontics, to remember how to do this, however I find this much more confusing then just learning how it works. I have a back grounding engineer, and surveying, so angles are second nature to me, but in this work I find I have trouble figure out when to use plus signs, and negative , and what the equations are. Further, I seem to become greatly confused when they say you have to "subtract "E", or something like that because to the east would be negative already. Further confusion exists because sometimes I feel like it is talking not about positive or negative but magnitude. So if for example I have deviation of - 2 degree west, and it ""decreases by 30 mins"" does that make it 2-30 or 1-30?

Can anyone either explain this to me, .. or point me to a reliable website that might make this clear?


Kevin.
True Virgins Made Dull Company, West is Best , Least is East

So when go from True to Magnetic to Compass
Virgins= Variation
Dull = Deviation
Therefore when going from True to Magnetic you Add West but Subtract East
and then when going further along from magnetic to compass you do the same
Variation you get from the compass rose
Deviation is different for every boat and every boat should have a deviation card
Cheers
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Old 01-02-2016, 00:52   #3
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Re: Variation and deviation, online Nav course, please help.

Time2go is correct.

Kevin just complete this table and everyone else keep off!

>>>>>> ADD WEST >>>>>>>

T V M D C
197 6w 4w
4e 122 4e
027 5w 4e
354 5w 2e
1e 086 5w
7w 2w 014

Whenever you need to consider variation & deviation just make a table and remember when working from left to right ie. from T through M to C ADD west

If you are working across the table from R to L minus west

Of course if you are dealing with E do the opposite.

When I press the post button I do not know if the table will reproduce as I have typed it??

It is not clear from your post whether you actually understand what V & D are. Perhaps if you posted your understanding of both we could correct any misunderstandings you have.

Mike
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Old 01-02-2016, 00:54   #4
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Re: Variation and deviation, online Nav course, please help.

As I suspected table did not work.

Perhaps someone who can use a computer who understands what I was trying to achieve could post a proper table

Mike
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Old 01-02-2016, 02:11   #5
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Re: Variation and deviation, online Nav course, please help.

OK, you know that thing on the chart which is known as the "Compass Rose", has the numbers around it going 000, 010, 020 030 etc.
That is normally showing True bearings, but in many cases will have a secondary north arrow indicating Magnetic North.
The difference between the True and Magnetic Arrow is your variation.
The variation is only correct at the position of the compass rose, and for the date stated . (It might show variation as 2W in 2001, and will also indicate the annual change of variation which might be 10' E)
In the example above, this would mean in 2016, the variation would have changed by 16 x 10 = 160'E, which is 2 Degrees and 40 minutes. So the variation for 2016 would now be 40 minutes East.

If you want to get a visual on how the variation works, imagine the True North Compass Rose, and then rotate that rose 40 minute east, (clockwise). What would have been previously 000 at the top of the rose, will now be 359 degrees and 20 minutes.

Clear as mud is it not?
Anyway, that was how I managed to get my head around the compass error problems many years ago.
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Old 01-02-2016, 07:14   #6
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Re: Variation and deviation, online Nav course, please help.

If you have, let say, a Magnetic bearing of 100M with a variation of 5E, the True bearing would be 100 + 5 = 105T. If you have a Magnetic bearing of 100M with a variation of 5W, the True bearing would be 100 - 5 = 095T.For changes from True to Magnetic the situations are reversed.
If you have a True bearing of 100T with a variation of 5E, the Magnetic bearing would be 100 - 5 = 095M. If you have a True bearing of 100T with a variation of 5W, the Magnetic bearing would be 100 + 5 = 105M.

The correction for magnetic variation for your location is shown on the nearest nautical chart's compass rose. So let say you have variation of 4 15' W in 2009, with an indicated annual correction of 0 08' E. Hence, in 2011 this variation is estimated to be 3 59', almost 4 West. This means that if we/you sail 90 on the chart (the true course), the compass would read 94.
Another example: compass rose gives a variation of 2 50' E in 2007, with a correction of 0 04' E per year. In 2009 this variation is estimated to be 2 58', almost 3 East. Now, if we sail 90 on the chart, the compass would read 87.
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Old 01-02-2016, 07:18   #7
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Re: Variation and deviation, online Nav course, please help.

Marine navigation courses: compass navigation
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Old 01-02-2016, 09:09   #8
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Re: Variation and deviation, online Nav course, please help.

Quote:
Originally Posted by Time2Go View Post
True Virgins Made Dull Company, West is Best , Least is East
Another mnemonic is
TV Makes Dull Children, Plus Waste
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Old 01-02-2016, 09:52   #9
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Re: Variation and deviation, online Nav course, please help.

Originally Posted by Time2Go
True Virgins Made Dull Company, West is Best , Least is East

I find it easier to remember - TVMDC, Error West compass best, Error East compass least, since it is the compass that is being corrected.

Roger
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Old 01-02-2016, 10:33   #10
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Re: Variation and deviation, online Nav course, please help.

This has always helped me:

Error west, compass best.
Error east, compass least.

There are many little phrases that really help. I say them out load, and now my kids know them too...even if they have no idea what they mean.

A few examples:

Green to green, red to red, perfect safety, go ahead.

If two lights you see ahead, starboard you wheel, and show your red.

If to starboard red appears, tis your duty to keep clear.

RED - HOT - POSITIVE (for wiring)

If in doubt, come about
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Old 01-02-2016, 11:04   #11
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Re: Variation and deviation, online Nav course, please help.

There are so many mnemonics it is certain confusing.

The classic one many have cited. How I learned it and then taught it for many years is the following:
First I would draw an arrow pointing in the direct of how converting the numbers, usually down because starting with the true course plotted from the compass rose outer circle, that helped me and thus students remember the direction. Yes the rhyme is True Virgins Make Dull Company, add Whiskey. And yes my apologies, please no one comment it is a silly rhyme.

Remember if the arrow you drew points down (which is most often the case because of plotting the course line from the outer circle of the compass rose) so you start with the first number on top (in this case of my example the True course) and add anything with a Whiskey (West) value and thus do the opposite (subtract) anything with an East value
T - True (true course) ex 125 degrees
V - Variation (given in the center of your compass rose) 12 degrees W
remember when following the down arrow add Whiskey
(add value if it is West, so do the opposite for an East
value when going doing and subtract it)
M - Magnetic (this is course after Adding Whiskey-West or subtracting
East value)
In my example T 125 degrees, V = 12 W, so Magnetic = 137 degrees
(add Whiskey)
D- Deviation (from your own compass card after "swinging") ex 2 degrees W
C - Compass course on your own boat is 137 + 2 (because it's W) so
Compass course on your boat is 139 degrees

When converting from True down to Magnetic and Compass (hence why when learning and even when doing the piloting on the boat, draw the arrow so you can remember which direction you are going) Fatigue has many times more than I wish to remember sneaked in and screwed me up, i suspect that's true for most of us.

Simplified. Going down from True always add anything with a West value, thus if East value, subtract.
Going up from Compass or Magnetic reverse it, subtract West, add East.

Going Down, Aye the Weasel - add West
Going Up, Excellent Choice - East it is (add East Values)

hope this is helpful


gwy - a warming up mainah!
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Old 01-02-2016, 11:24   #12
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Re: Variation and deviation, online Nav course, please help.

Kevin? Are you totally completely confused yet? I just completed and passed my ASA 105 and reading all that I am confused all over again...
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Old 01-02-2016, 11:27   #13
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Re: Variation and deviation, online Nav course, please help.

Try this one TVMDC – Practice – Correcting and un-correcting the compass | Boating Safety Tips, Tricks & Thoughts from Captnmike
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Old 01-02-2016, 12:38   #14
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Re: Variation and deviation, online Nav course, please help.

Just a quick one here... this "add west" etc is not that easy to remember..

This is:

True .......... True
Virgins ....... Variation
Make .......... Magnetic
Dull ............ Deviation
Company ..... Compass
At .............. Add
Weddings .... West

Can............. Compass
Dead ...........Deviation
Men ............ Magnetic
Vote ........... Variation
Twice ......... True
At .............. Add
Elections...... East

Once you learn it, you will never forget it. just write it down as you say it, then fill in the numbers (you have to be aware of the obvious.. if it says Add west then you must subtract east..)

M
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Old 01-02-2016, 18:26   #15
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Re: Variation and deviation, online Nav course, please help.

Variation is the offset of magnetic north to true north. You will see it on the compass rose of your chart. This offset a function of geography. Deviation is the error of your compass. It may be + or - and can be influenced by iron on the boat or nearby electronics or current carrying wires. Don't be confused. Remember, the 'Deviants' are on the boat.
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