Magnetic vs. True North

As I understand it, true north does not vary and it coincides with the spin axis (geographic north). Magnetic north varies with time in magnitude and orientation due to changes in the earth's magnetic field.

The two are very important if you are navigating in the wilderness with a map and compass. You should account for magnetic declination (the difference between geographic north and magnetic north).

Magnetic north is the point on the earth's surface where the north magnetic pole of the earth's magnetic field "emerges" from the earth. This point is not fixed, but moves or shifts as the earth's magnetic field changes. It moves, and it should be noted that the polarity of the magnetic field has abruptly "flipped" or changed direction many times over the billions of years the earth has been here.
True north is that point on the northern or "upper" part of the earth where the earth's axis of rotation meets the surface of the earth. This is also called the geographic or terrestrial north pole. The difference is essentially one of the astronomical characteristics of the earth, that axis of rotation thing, and one of the earth's geophysics, the earth's magnetic field and its unstable nature.

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True north is a constant and refers to the geographic North Pole. Magnetic north tends to shift and refers to the pole of the Earth's magnetic field. In mid 2002, true north and magnetic north were approximately 590 miles apart.
This handy article from hiking expert Doug Latimer explains the poles of the Earth's magnetic field are different from its geographic poles. Maps are aligned along true north, so hikers have to make adjustments when navigating by compass.

In navigation, the difference between true north and magnetic north is known as declination. All U.S. Geological Survey maps print relevant declination information, and the maps are updated every five years to account for shift. Hikers traveling in Northern California, for instance, have to make declination adjustments of roughly 18 degrees.

The Earth's magnetic field stems from its molten metallic core, much of which is iron. Iron is a fairly common element, since it can't be burned off during the fiery formation of stars. Iron is magnetic because its inner electron shells are slightly unstable.

As Seneca said, true north doesn't vary and is the spin axis. Magnetic north varies over time as well as with physical location!

When using a magnetic compass (hand-held for example) it's clearly important to 'calibrate' it for the variance in magnetic field for the location where you are using it. It's called 'declination' and a decent magnetic compass has markings on it that allow you to turn the whole dial to reflect how far in degrees, and in which direction the magnetic field deviates from true north.

The NOAA web site has an explanation of it here:
http://www.ngdc.noaa.gov/geomag/declination.shtml

And a calculator for the present time and your location here:
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp

(Ha! Frside007 & I must have been replying at the same time! )

If you look at a globe or a map the longitude lines converge at True North. For the casual user, a magnetic compass points to Magnetic North and that is generally good enough. As stated earlier, if you're in the bush and have a map and a compass, it is a good idea to consult the declination chart to compensate else you wander the wrong way.

"True North"
The location of the north pole,on the surface of the earth relative to one's location.

"Magnetic North"
The direction to which a compass points. Magnetic north differs from true north because the magnetic fields of the planet, and not exactly in line with the north and south poles.

Observed differences between magnetic and true north is known as magnetic variation.

The question was: "What's the difference and how does it affect me between "True North" and "Magnetic North"?".

The "difference" has been amply answered but the "how does it affect me" has been skipped.

So to finish answering the original question; If you are using a GPS then you will not likely be affected by the difference.

I might add that his is because the GPS determines direction of travel from satellites rather than magnetic fields.

The magnetic north and true north would be more of a concern if you were hiking or out in the backcountry. While driving on roads and highways with City Navigator it's a non-issue unless you just have to know what direction you are travelling.

In reading this and thinking about it, wouldn't the only real need for Magnetic north on a GPS just be for grins, giggles, and another item in the feature list?

If you have a GPS you can set a destination for where you want to go and it will just route you there, north or magnetic north same route. The only use I could think of for magnetic north would be if you wanted to use the GPS as a standard map and you had a physical compass you were trying to navigate with.