magnetic declination?

Answer 1

The magnetic declination at any point on the Earth is the angle between the local magnetic field -- the direction the north end of a compass points -- and true north. The declination is positive when the magnetic north is east of true north. The term magnetic variation is equivalent, and is more often used in aeronautical and other forms of navigation. Isogonic lines are where the declination has the same value, and the lines where the declination is zero are called agonic lines.

Magnetic declination varies both from place to place, and with the passage of time. As a traveller cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas), meaning a compass adjusted at the beginning of the journey would have a true north error of over 30 degrees if not adjusted for the changing declination.

In most areas, the spatial variation reflects the irregularities of the flows deep in the earth; in some areas, deposits of iron ore or magnetite in the earth's crust may contribute strongly to the declination. Similarly, secular changes to these flows result in slow changes to the field strength and direction at the same point on the Earth.

The declination in a given area will change slowly over time, possibly as much as 2-25 degrees every hundred years or so, depending upon how far from the magnetic poles it is. This may be insignificant to most travellers, but can be important if using magnetic bearings from old charts or metes (directions) in old deeds for locating places with any precision.

all the best.

Answer 2

I guess that you are asking what is it.
Magnetic declination is the difference between true north and magnetic north. True north would be the axis point or Pole where Magnetic north is roughly 450 miles from the true pole, although throughout time this has and will change due to the rotation of the planet and solar influence