A question about Magnetic Deviation...

[ddavid 149]

Joe's Guess the Airport clues for Z11D included a magnetic Variation of - 6.1 degrees (negative). Here's the edited airport plate:

I wasn't sure where negative Magnetic Variations were, other than in Greenland, so I looked it up. Here's a nice image showing its distribution worldwide:

The red lines are positive and blue lines negative. So I was surprised when the answer came up in Finland! What does Plan-G say, I thought. Well, Z11D comes up like this:

Magnetic Variation at Z11D is 7.4 East - positive or negative?

So, what do you think? I'm confused - can you help?

Cheers - Dai.

[Tim_A 997]

It's confusing, because those magdec charts work backwards compared to how the data is used in real life. On the chart, the blue numbers are -ve, representing West (the difference from magnetic to true), but in day to day use, you always want to go the other way (i.e. the angle from true to magnetic). So on charts, and in FS and Plan-G you'll see West as +ve, and East as -ve. In the UK it ranges from about 2 degrees in the South East to over 6 degrees in the West of Ireland. So, to go from true north on the chart to magnetic north on the compass, we add the westerly variation.

You'll sometimes see the figure quoted for magnetic declination. Declination and variation are synonymous, and defined as the angle between Magnetic North and True North. The Earth's magnetic core is constantly moving, and as result, magnetic variation is also constantly changing (you'll see the rate of change printed on charts).

A table is published ever five years (2000, 2005, 2010, 2015...), of the ' World Magnetic Model'. Actually it's a sophisticated set of equations in addition to several tables. It's a joint publication of the British Geological Survey in Edinburgh and the US National Geophysical Data Center in Boulder Colorado. FS9 is based on WMM2000, FSX on WMM2005. Plan-G also uses WMM2005. The data in FS is stored in a file called magdec.bgl. it's a simplified table, recalculated in half-degree blocks. Plan-G calculates magdec on the fly using the full equations, and displays it 'live' in the Info window, updating as you move the mouse. For greater accuracy (in theory anyway), FS also has a variation value stored with every airport and navaid. For the default airports this comes from Jeppesen or ICAO chart data (which may or may not be accurate, particularly when it comes to small rural strips). For addon airports, well, who knows...

You'll sometimes come across the term magnetic deviation. This is something different. It's the difference between magnetic North and compass North. This is different for every compass, and accounts for magnetic fields from metal housings, wiring, electric fields, even the hull of the boat/plane. A calibrated (or 'swung') compass will include a deviation card, which gives the correction factors at different headings. You'll see this card in most FS aeroplanes, in a little holder just above the compass.

[ddavid 149]

Thanks, Tim - I think I'm getting the gist. My problem is, as you say, when you've got a compass in your hand. Which way is positive/negative - to the east or to the west?

And here's another Magnetic Declination image - from 1995:

This also shows northern Finland as positive, so do I automatically 'negate' the value, as in Joe's original airport plate, and make the minus 6.1 into plus 6.1? Doh!

Cheers - Dai.

[Tim_A 997]

Yup. The images all assume you start with a magnetic bearing and want to calculate true. But in aviation we always start with a true bearing, and want to calculate the magnetic. So you need to reverse the sign.

[mutley 4,498]

I knew that! :lol: ?not?

Thanks Tim and Dai for the info, the GTA that lives on forever