If you use a compass as well to maintain a heading, magnetic north will most likely suit you the best.

the magnetic field is always changing......correct; but I'm no physicist so cant explain in detail.....but it has always changed through out life, even to the point of a complete reverse in polar magnetic fields (ie. the north and south pole completely flip/revers to opposite polarity and we are overdue for such). got to do with all the magna/molton lava/iron flow sub tectonic plate movement.....but here goes after two rums......

SAILORS have long known that the magnetic North Pole wanders about a lot but it could be the ocean currents making it happen.

Controversial new research suggests the ebb and flow of the seas are responsible for changes in the earth's magnetic field.

The findings challenge accepted ideas about how the motion of molten iron in the earth's core produces the magnetic shield that is vital to protect us from charged space particles. The study, by US researcher Gregory Ryskin, has been criticised by some scientists.

But the claims would stimulate new research, whether Professor Ryskin was right or wrong, said Eric Tohver, a lecturer in the School of Earth and Environment at the University of Western Australia.

The earth is thought to have a solid inner core that spins inside an outer liquid metal core. The movement generates the magnetic field that reaches into space.

Fluctuations in the field over decades, such as the changing position of magnetic north, are said to be due to convection in the liquid outer core, as the hotter molten iron moves outwards, cools and then sinks back.

Ocean currents flowing through the earth's magnetic field create their own magnetic fields.

Most scientists have dismissed this seawater effect as a weak one, but Professor Ryskin, of Northwestern University in Illinois, showed in a paper in the New Journal Of Physics that the field created by all the ocean currents would be big enough to cause the fluctuations in the global magnetic field. Most controversially, he questioned whether the molten core was moving at all.

interesting idea.......

mag north allows for this change over time.

The direction in which a compass needle points is known as magnetic north. In general, this is not exactly the direction of the North Magnetic Pole (or of any other consistent location). Instead, the compass aligns itself to the local geomagnetic field, which varies in a complex manner over the Earth's surface, as well as over time. The angular difference between magnetic north and true north (defined in reference to the Geographic North Pole), at any particular location on the Earth's surface, is called the magnetic declination. Most map coordinate systems are based on true north, and magnetic declination is often shown on map legends so that the direction of true north can be determined from north as indicated by a compass.

most GPS systems will have the coefficient already programmed. paper maps on the other hand have a decriptive provision for the calculation of mag north on the map and you just work out the difference from the date and scale of change per year according to your position on the globe........ the map will state the difference and change over time according to your global position on the map itself so you just work it out accordingly and set the difference onto the compass by manually changing the compass dial........ if you dont allow for this difference then if you use a cross bearing from a map to set a bearing you will not arrive at this precise destination....... 

you generally buy three types of compasses....one for the northern hemisphere which is calibrated to the strength of the north pole magnetic field, one for the southern hemisphere which once again is specifically aligned for use in the southern hemisphere as the strength of the magnetic field will be less than that of the norhtern hemisphere, and the third is a universal compass which can be used in either hemishere which is obviously calibrated at a more sophisticated level and is the most expensive and sensitive.

so you finally bought the 585.........

still ain't gona get you some snapper...ha ha

Would be pretty interesting if the ocean currents do affect the magnetic field, but then how would it explain pole reversal in the case where the core was still? The dynamo theory states that the core must be moving in order to generate the field though, so i'm not sure why he's mentioning that it could be fixed.

As for the compass types, I think it is more to do with the balancing of the needle based on the declination of the magnetic field within a certain region, not the strength of the field itself. Theres a nice map here of the variations in magnetic field over the earth: http://en.wikipedia.org/wiki/File:Magnetic_Field_Earth.png. It works out to around 1/2000th variations though, so its pretty uniform over the whole earth.

BJ: if you're using the gps/chartplotter for directions use true north, otherwise as stomper said, use the magnetic one (because then it will match the bearings a bit more). You can find out the different at different locations here: http://www.ga.gov.au/oracle/geomag/agrfform.jsp

bit of everything Matt,

Compass balancing

Because the Earth's magnetic field's inclination and intensity vary at different latitudes, compasses are often balanced during manufacture. Most manufacturers balance their compass needles for one of five zones, ranging from zone 1, covering most of the Northern Hemisphere, to zone 5 covering Australia and the southern oceans. This balancing prevents excessive dipping of one end of the needle which can cause the compass card to stick and give false readings

Did you cut and paste that from wikipedia? :p

i understood nothing in that whole thing... plz type in english next time :D