Variation and deviation +/- convention

[VicS]

A recent thread has set me wondering.

I have always used east / west notation for variation and deviation but what is the convention when its expressed purely mathematically.

If you are told the variation is +2° does that mean 2° west or 2° east.

Nothing is coming off my bookshelf to tell me.

 

[sarabande]

Positive is Easterly.


Currently the UK is almost agonic (neither + nor -)

 

[lw395]

I would expect + variation to mean the compass read higher than true.

I would also expect some people to disagree....

 

[AngusMcDoon]

If you are told the variation is +2° does that mean 2° west or 2° east.

Nothing is coming off my bookshelf to tell me.

East. See here...

http://magnetic-declination.com/

West is negative in Seatalk messages too.

As usual, Wikipedia is the font of all human and cat knowledge...

Example of magnetic declination showing a compass needle with a "positive" (or "easterly") variation from geographic north. Ng is geographic or true north, Nm is magnetic north, and δ is magnetic declination.

 

[alanch]

I have never heard of variation referred to as + or -, only ever east or west as shown on charts. Whether you + or - east or west depends what you are trying to do - go from compass to true or true to compass.

 

[AngusMcDoon]

Positive is Easterly.


Currently the UK is almost agonic (neither + nor -)

-5 in the Ooter Hebrides earlier today.

 

[AngusMcDoon]

I have never heard of variation referred to as + or -, only ever east or west as shown on charts. Whether you + or - east or west depends what you are trying to do - go from compass to true or true to compass.

That may the way with paper chart navigation, but inside marine electronics it is either stored as a signed number or calculated from a world model, usually the International Geomagnetic Reference Field Model...

http://www.ngdc.noaa.gov/geomag/declination.shtml

The model uses - for W and + for E, and that's the way it is always stored as far as I have seen.

 

[roblpm]

-5 in the Ooter Hebrides earlier today.

Penzance seems to be the place to be. Then it can be forgotten for a while!

 

[alanch]

That may the way with paper chart navigation, but inside marine electronics it is either stored as a signed number or calculated from a world model, usually the International Geomagnetic Reference Field Model...

http://www.ngdc.noaa.gov/geomag/declination.shtml

The model uses - for W and + for E, and that's the way it is always stored as far as I have seen.

Thanks for that. Thats interesting. Unfortunately my navigation education ended in 1976 when I took my last professional ticket, and I have failed to keep up! Since then the mark 1 eyeball out the bridge window (or on the radar in fog) has continued to serve me well. Luckily, only 4 more months to retirement.

 

[AngusMcDoon]

Penzance seems to be the place to be. Then it can be forgotten for a while!

Still nearly 3 degrees in Penzance. Ostend it's a big fat zero.

 

[doug748]

Analogue rotary controls turn up (+) the volume when tweaked to the right. Conventional right hand screwthreads tighten in the same manner, so it is all in God's plan.

The only method of remembering the Deviation/variation thing, that I ever used was:

Compass to true the signs will do
Compass to worse the signs reverse

Which depends on this convention: - West +East

 

[sarabande]

variations (sorry ! ) in + or - all over the UK.

http://magnetic-declination.com/

 

[AngusMcDoon]

Thanks for that. Thats interesting. Unfortunately my navigation education ended in 1976 when I took my last professional ticket, and I have failed to keep up! Since then the mark 1 eyeball out the bridge window (or on the radar in fog) has continued to serve me well. Luckily, only 4 more months to retirement.

What the leisure marine electronics manufacturers fail to mention when they sell you a shiny new chart plotter is that the magnetic variation model from the last software update until you finally throw away the device will be slowly but continuously going out of date and becoming less accurate. After a couple of decades it may be a few degrees out. This may not bother anyone, but it's worth being aware of it. The model is only a prediction of what's going to happen in the future; there is no certainty about it at all. Commercial software is updated throughout its life so does not suffer this problem.

 

[sarabande]

what happens e.g. when you do a s/ware update for a nav unit ? Will the manufacturer automatically update the variation and tell you ? (I don't have any such pieces of kit )


And in any case what happens if someone buys a plotter in Penzance, and sails to Burnham or yer Ooter Hebrides ? Are all outputs True and if so how does the sensor know how to change the + or - ?

 

[AngusMcDoon]

what happens e.g. when you do a s/ware update for a nav unit ? Will the manufacturer automatically update the variation and tell you ? (I don't have any such pieces of kit )


And in any case what happens if someone buys a plotter in Penzance, and sails to Burnham or yer Ooter Hebrides ? Are all outputs True and if so how does the sensor know how to change the + or - ?

It's not the variation at one location that is updated, it's the world model (if there's a new one available). I know one manufacturer does this. Can't speak for others.

If lat/long is available then variation and hence true heading can be calculated from the world model. Say, for example, you had a B&G N2K compass sensor and a GPS transducer. Both of these would stick their data on the N2K bus. A Zeus plotter or a Triton display would receive these values and from them and its world variation model in its software calculate variation for that day and that position, and stick that data back on the N2K bus. It's all jolly hockey sticks geekery.

 

[sarabande]

TA ! It's something I had not thought about

 

[ghostlymoron]

Bang on!

I would expect + variation to mean the compass read higher than true.

I would also expect some people to disagree....

 

[lw395]

Bang on!

Looks like most people would disagree with my expectation.
I blame the influence of those merkin types who put their green booees on the other side....

In my work, there are lots of cases like this 'errors ' and 'corrections' and 'factors', it never pays to assume people using the 'obvious' definition.

 

[VicS]

Looks like most people would disagree with my expectation.

I was thinking pretty much the same way as you were .... seems we were wrong though.

Best stick to east and west I think at least we know which of those is which ........ until the N & S poles next swap over.

I certainly dont get along with all the mnemonics, rhymes and other silly jingles .....

Virgins to the west and Mecca to the right ......... ???

 

[KellysEye]

>I have never heard of variation referred to as + or -, only ever east or west as shown on charts.

Agree.