Slightly confused about Magnetic Variation

**LuckyBlundy** · 05-04-2012, 02:33 AM

I just finished reading "The Spirit of St. Louis" by Charles Lindburg in which he gives the navigation logs for his Atlantic Flight. All the compass deviations he used were W, which I take to be positive. He added them to his True Course to obtain his compass course (he also adjusted for wind and magnetic deviation but I'm only interested in mag variation). Of course the numbers are different now, that was in 1927.

I used the Earhart Lockheed Electra to find the mag variation at LAX, which was 14 degs. W(positive), however, when I checked it out to see if I was doing it correctly, I found that I had to subtract 14 degrees from my desired true course in order to get the correct compass heading. I checked it again by going to JFK (which has a negative variation, E) and found that in order to fly my desired course I had to add the variation to get the compass course.

AT LAX: Desired Course 0 degrees true north. Variation +14 degrees. Compass course 0-14 = 346 degrees.
AT JFK: Desired Course 0 degrees true north. Variation -13 degrees. Compass course 0+13 = 13 degrees.

It seems to me that the signs are backward, or is my thinking screwed up? And how did Lindburg know what the mag var was at any given point?

**NikeHerk67** · 05-04-2012, 04:35 AM

I’m not sure if these examples, of the way I think of it, will help or confuse you more.

I think of True North as always being the constant, and Magnetic Variation, as always being the variable.

At LAX when heading magnetic 360 or 0 degrees you're actually heading +14 degrees from true north
Or, when you’re flying Magnetic 360 degrees you’re actually flying +14 degrees true. To fly true north (0 degrees) you must turn LEFT -14 degrees which is the variation at that location.
(Years from now it may only be 12 or 13 degrees). It varies.

The Agonic Line is an imaginary line where magnetic north and true north are the same and is located in the USA just east of Chicago as example. The Magnetic pole is true north from there in northern Canada near the Arctic Circle.

When taking flying lessons in western PA with a magnetic variation of -8.1 degrees my instructor would request me to fly true north, east, south, or west just to keep me aware of where I was really heading on the map. That was years and years ago, I'm sure it's changed by now...probably less as the magnetic pole is moving more north.
I could visualize heading to northern Canada somewhere above Chicago so I always knew that I needed to turn RIGHT to a heading plus 8 degrees from the compass.

To add to the confusion we had a compass deviation card that showed us the calibrated compass headings for every 30 degrees. It didn’t take too long to memorize that information on the card. The compass might only be accruate at two headings and vary from plus or minus a few degrees, usually not more than about 5 degrees or so.

As example, if, the card had on it for 90 degrees to steer 87 degrees then to head due east I’d need to set the compass to 87+8 or 95 degrees to head due east.

One more example: It may help you to draw a line from the magnetic north pole to LAX and then to JFK on a map. When heading 0 degrees on the compass you're following that line. Then it's easy to tell which way you need to turn to true north. From there it's the same for each heading all around the compass.

**Iceman07** · 05-04-2012, 08:45 AM

It seems to me, at least, that you are thinking about this "backwards".

When doing your flight planning, most of the information you are working with is given relative to True North. i.e. The winds aloft are given in True North. When you draw your line on the sectional, and then measure the course relative to the Lat/Long lines, the resultant is relative to True North. Only after you determine True Heading do you then add or subtract the Magnetic Variation. In real life, (VFR flight planning, anyway) the Magnetic Variation is determined by the (as NikeHerc said) the "Agonic Lines" on the chart. See the following link. The Agonic lines are the dashed magenta lines that run at angles from the top to the bottom. The Magnetic Variation around Long Island, Connecticut and that area is 14 degrees West.

http://vfrmap.com/?type=vfrc&lat=40....72.867&zoom=10

Once you get the winds aloft, you use your flight computer (E6B, calculator, etc.) to determine your Wind Correction Angle.

Then, you use the following to ultimately determine your "Compass Heading"

TC (-E Var/+W Var) = MC MC (+/-) WCA = MH MH (+/-) Dev = CH

TC = True Course
Var = Magnetic Variation
MC = Magnetic Course
WCA = Wind Correction Angle
MH = Magnetic Heading
Dev = Magnetic Deviation (Obtained from the deviation card in that particular aircraft)
CH = Compass Heading (This is the actual heading you will fly)

For example:
Given:
True Course = 65 Degrees
Magnetic Variation in that area is 14 degrees West
Wind Correction Angle = -8 Degrees
Magnetic Deviation = -2 Degrees

65 + 14 = 79
79 - 8 = 71
71 - 2 = 69

So, when you get into the airplane, start up and take off, your initial Compass Heading will be 69 degrees.

**Iceman07** · 05-04-2012, 09:16 AM

BTW - the Magnetic Variation at LAX is 12 Degrees, 30 Min. East.

From what I understand from your original post, you are planning a flight from LAX to the North Pole.

Given:

True Course = 0 Degrees
Magnetic Variation = -12.5
Wind Correction Angle = 0 (assuming no wind)
Magnetic Deviation = 0 Degrees

0 - 12.5 = 347.5
347.5 - 0 = 347.5
347.5 - 0 = 347.5

This would be your initial compass Heading if you wanted to fly to the North Pole (True North Pole, that is, not the magnetic North Pole)

As Nikeherc said, if you wanted to fly to the Magnetic North Pole, you would (in theory, anyway) fly the Line of variation. Assuming no wind and no deviation, if you fly a compass heading of 0 degrees from LAX, you will (eventually) reach the magnetic North Pole.

**Iceman07** · 05-04-2012, 09:19 AM

Originally Posted by LuckyBlundy

And how did Lindburg know what the mag var was at any given point?

The lines of Magnetic Variation were well understood and mapped by that time. Ships navigation also uses them. Lindbergh used existing nautical charts to determine the Variation.

**tommchowat** · 05-04-2012, 09:34 AM

There's a little phrase that my Instruments instructor tought me during ATPL ground school: Cadburys Dairy Milk Very Tasty, or CDMVT.

C - Compass heading
D - Deviation
M - Magnetic heading
V - Variation
T - True heading

Two rules are:

1) Deviation West, Compass Best (if the deviation is West, you add it to the Magnetic heading to get the Compass heading & vice versa)
2) Variation East, Magnetic Least (if the variation is East, you subtract it from the True heading to get the Magnetic heading & vice versa)

Using this, you can work out magnetic and compass headings using just a true heading and values for variation and deviation, or a true heading from a compass heading and values for variation and deviation.

Example:

Your true heading is 140 degrees. Variation is 12 East and deviation is 2 West. What is your compass heading?

Put the numbers into CDMVT:

C - Unknown just now
D - 2 West
M - Unknown just now
V - 12 East
T - 140 degrees

Working from T to C, we can get the numbers:

T - 140 degrees
V - 12 East
M - Variation East, Magnetic least, so 140 - 12 = 128 degrees
D - 2 West
C - Deviation West, Compass best, so 128 + 2 = 130 degrees

So, to answer the question, if your true heading is 140 degrees, with 12 East of variation and 2 West of deviation, your compass heading is 130 degrees.

Simples! These do assume no wind, so as said above you can work out your wind correction angle in the way that you like

**teson1** · 05-04-2012, 11:10 AM

I believe LuckyBundy was just confused by interpreting a magnetic variation of "+14°" indicated by his source as "14° W".

However, the definition of a signed variation/declination value is that a positive value (+) means easterly variation (E), and a negative value (-) means westerly (W) variation, i.e. "+14°" means "14° E".
http://en.wikipedia.org/wiki/Magnetic_declination

As far as I remember if magnetic variation is read within FS by variable (A:MAGVAR,degrees) FS seems to use this definition where "+14°" means "14° E", and "-2°" is "2° W" .

So the rule "East is least - West is best" for going from true to magnetic course is only correct if using the absolute value of variation ("14°" W, "2°" E).

If using the signed value of magnetic variation, the calculation is
MC = TC - MV
(Example: TC=0°, MV=+14° -> MC=0°- "+14°"=346°)

Sorry US guys - West is not always best

BR

**mgh** · 05-04-2012, 11:39 AM

Originally Posted by teson1

So the rule "East is least - West is best" for going from true to magnetic course is only correct if using the absolute value of variation ("14°" W, "2°" E).

Had you not noticed the use of the words East and West in the rule?