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magnetic declination — English
The angular distance between the geographic north pole and the Earth’s magnetic north pole. The magnetic north pole moves about over a relatively small area on the Earth’s surface in a process called “pole wandering”, while the geographic north pole remains in one and the same position. A compass needle will always point to the magnetic north pole, not the geographic one. Consequently a bearing taken by means of a compass has to be “corrected” to know where true north is. In order to make the correction, one has to know what the magnetic declination is (see “bearing”). To complicate matters, polar wandering is a relatively fast process and the magnetic pole perpetually shifts its location. To make it even more complicated, magnetic declination differs from place to place because the direction to the magnetic pole as measured at one place on the Earth’s surface will differ from that measured from another place. Fortunately all official, published maps (such as the 1:50 000 topocadastral series of South Africa) display a little diagram on it margin to indicate the declination from that area at the time of the publication of the map. With this information it is easy enough to establish where true north is (see “bearing”), but without this information we could not establish exactly where true north is. Modern day navigation instruments (such as GPSs) provide this information, but when you are lost somewhere in the wide outdoors with a compass as your only navigational tool, you will only be able to establish a general north direction, not an exact one.
