Since the North and South Magnetic Poles are not located exactly on the Earth's North and South Poles, respectively, and due to other irregularities in the Earth's magnetic field, true north differs from magnetic north in most locations. One consequence of this is that magnetic compasses do not point to true north in most places. Magnetic declination is the angle between the local magnetic field and true north, and can be represented on maps by isogonic lines, which connect locations that have equal magnetic declination.
From
Wikipedia: Magnetic declination:
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 magnetic declination in a given area will change slowly over time, possibly as much as 2-2.5 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. Simply speaking, true north is the direction in which the north pole is located along the Earth's rotational axis.
On the other hand, magnetic north is the direction toward which the compass needle points.
Magnetic declination described by signed degrees. Map from
Wikipedia: Magnetic declination.
Also see
Google Earth Community: Earth's magnetic field and declination, 1590-2010.
The attached KMZ file indicates magnetic declination for the 48 contiguous United States. The paths on the map are isogonic lines traced from the
NIMA Magnetic Variation Map 2000, which was downloaded from the Wikipedia article cited above on August 25, 2009. Accessory data in the KMZ file also includes United States state boundary data downloaded in shapefile format (admin98.shp) from the geodata page at
Mapping Hacks. This helped to align the NIMA overlays for various parts of the United States, which was necessary because the map was not in plate carree format, which is ideal for creating overlays in Google Earth.
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