What is the difference between a true and a magnetic heading?
Why are/were magnetic headings used in aviation?
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asked Dec 18, 2013 at 16:19
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6 Answers
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The "heading" refers to the direction an aircraft is pointing. For a Magnetic Heading, this is in relation to Magnetic North. For a True Heading, this is in relation to True North.
True North is directly over the earth's axis.
Magnetic North is somewhere over Canada, moving towards Russia.
To get the Magnetic Heading, you just read it off the magnetic compass.
To get the True Heading, you need to first read the magnetic compass, then either add an Easterly, or subtract a Westerly, magnetic variation; based upon the isogonic lines on your sectional (the purple dashed lines labeled 5°W, 3°E, etc).
Example 1: Magnetic Heading 177 w/ 3 degrees East Magnetic deviation = true course 180.
Example 2: Magnetic Heading 177 w/ 3 degrees West Magnetic deviation = true course 174.
Because of this, in the past, magnetic headings were used because a simple compass could be used. Finding reliable true headings was difficult until the era of things like the gyrocompass (patented in 1906 (Germany) and 1908 (USA)) and more recently, GPS.
answered Dec 18, 2013 at 16:30
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2$\begingroup$ You also need to take into account the compass deviation table which (hopefully) compensates for built-in magnetic influences. $\endgroup$ Dec 18, 2013 at 16:59
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$\begingroup$ Here's a question and answer what happens when you follow a constant magnetic heading due to compass deviation:aviation.stackexchange.com/questions/13248/… $\endgroup$– sweberMay 3, 2015 at 9:55
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1$\begingroup$ Note that gyrocompass only works when stationary or moving slowly, so while it existed in the early days of aviation, it couldn't be used on aircraft. Only with introduction of inertial navigation systems aircraft got gyroscopes good enough to align before departure and then maintain the true north with useful precision during the flight. $\endgroup$ Apr 23, 2016 at 21:22
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1$\begingroup$ Also GPS alone can't tell you true heading, because it can only calculate position and from it true track, but that differs from heading by (unknown) wind. What it can be used for is improving precision of INS. $\endgroup$ Apr 23, 2016 at 21:23
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True North is the North Pole. The maps used for navigating are oriented to the North Pole. A pilot can measure the direction between two points to create a 'track' or 'course' to fly in degrees true.
Magnetic North is where the compass points - which is not actually the North Pole. Magnetic North is currently over northern Canada.
The angular difference between True and Magnetic north is called Magnetic Variation. This number varies depending on where you are on the earth.
Magnetic Variation is the bugbear of many student pilots. Why is it important? Well if your track to fly (ascertained from a map) is 360 degrees true in relation to the North Pole, you should be flying right to the North Pole. But 360 degrees on the compass points to Magnetic North in Canada. If you haven't applied magnetic variation, you could be flying as much as 15 degrees or so off course.
Runway names are the headings given in degrees magnetic, but as magnetic north is slowly moving, occasionally the runway name (eg 35/17) will change.
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The true north pole, aka the celestial north pole, is the point on the Earth's surface intersecting Earth's rotational axis on the northern hemisphere (and thus the axis around which all stars appear to rotate). Prior to the introduction of the Global Positioning System, there were no perfect indicators of true north.
Two competing and often conflicting definitions of north have been used prior to the advent of GPS (and since). Magnetic north is the direction in line with the Earth's magnetic field, thought to be caused by the convective flows of liquid iron in the Earth's outer core, which causes a compass to point toward the magnetic north pole. An approximation of "celestial north" is in the direction of Polaris, which is a fairly bright star in the night sky and also the closest such star to Earth's rotational axis (and thus "true north") for about the last 1500 years.
First, the two poles do not coincide; here's a Google Earth image showing true north in green and magnetic north in red:
As you can see, they're close, especially from the perspective of those of us in the New World, but if you were sailing off the west coast of the British Isles, you might see up to a ten-degree difference between your compass and Polaris, and that's significant over a distance of even a few hundred miles. Navigators have known of this difference, known as the "magnetic declination", for centuries, and it was a secondary driving reason for the solution to the "problem of longitude" (how to determine, quickly and accurately, your current longitude; knowing this, in addition to being a key coordinate of your position at sea, also allows you to determine the necessary amount of "declination" from magnetic north to determine true north and thus the correct magnetic heading to your intended destination).
Second, Earth's magnetic pole changes over time; here's a map showing the changes in the surveyed position of the magnetic north pole through Nunavut, Canada over the past 130 years or so:
As you can see, the magnetic pole appears to be approaching true north, possibly due to gyroscopic stabilizations of the convective flows within earth's liquid inner layers. Again, for most of the U.S. the practical effects of observed shifts are minimal, but it has much more pronounced effect near the Prime Meridian which happens to be nearly perpendicular to the line between true and magnetic north.
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2$\begingroup$ Actually, on the west coast of USA, the effect is much more pronounced than in UK. In London, magnetic variation is a little less than 1°W while in Seattle it is almost 16°E! Because the magnetic field is not very regular. See en.wikipedia.org/wiki/File:World_Magnetic_Declination_2015.pdf $\endgroup$ Apr 23, 2016 at 20:44
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True north (geodetic north) is the direction along the earth's surface towards the geographic North Pole.
True geodetic north usually differs from magnetic north (the direction a compass points toward the magnetic north pole), and from grid north (the direction northwards along the grid lines of a map projection).
Magnetic declination is the angle between compass north (the direction the north end of a compass needle points) and true north (the direction along the earth's surface towards the geographic North Pole).
So to get Magnetic Heading, look at your compass. To get the true heading, you will need to look to a WAC chart, and depending on your position, add or remove degrees according to your position's Magnetic declination .
answered Dec 18, 2013 at 20:05
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Magnetic heading works with a compass. No need for electricity, no need for a radio, no need for satellites. If you have a failure of any navigation system, or main battery, your compass will tell you the magnetic pole (which is slightly different than the real pole, depending where you are in the globe). It's a safety feature. Satellite work great, when they work, and when your navigation system works. If your battery is dead or something get disconnected, you need some backup that works! You can't say "I lost my battery fuse for my navigation, therefore I got lost and we are going to die" you need a backup, that's why.
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True heading is true course ( as plotted on the chart) corrected for the wind. Period. Plus or minus magnetic variation gives you a magnetic heading, then compensation for compass deviation gives you your compass heading. Remember the old adage?...True Virgins Make Dull Company. Works every time.
