axis of rotation. The lines of longitude on maps are in true North/South directions. Magnetic north and variation The Earth has a magnetic field which is not quite in line with the geographic directions. The difference between the magnetic and true directions is known as Variation. It differs from place to place and changes with time. Large scale charts and maps will usually include a compass rose showing variation. Compass north and deviation The compass will indicate the magnetic field direction at the place where it happens to be at the time. If there are influences other than the Earth’s magnetic field, these may change the direction indicated by the compass. These effects are called deviation, and can be caused by a whole range of things. Any magnetic object or electrical current will have an influence, some more than others. The current in a dive computer is too small to affect the compass, even when quite near, but the hull of a ship or overhead power lines may make a difference even several meters away. It is difficult and often impossible to correct for all possible deviations, but it is worth checking a dive compass for deviation caused by dive equipment. It has been known for regulators to cause deviation, steel cylinders can cause deviation, and powerful lights may be a problem. A diver propulsion vehicle with an electric motor is also a potential problem for those who use them, though divers have been known to navigate adequately using compasses mounted on the handgrip of a DPV. A magnetic clip used to secure equipment to the diver's harness has a powerful magnet in both parts, and should not be used to hold the compass, as the part attached to the compass will produce a serious error. Deviation may be checked by comparing the compass bearing as measured with a known magnetic bearing measured by a compass with no deviation. Deviation may vary with different directions and for accurate work it is necessary to make up a table of deviations. This is done for ships, but for diving it is generally not worth the trouble. Bearings of one diver's compass may vary from those of another diver even if they have both been read correctly. The difference should not be large, but it can result in being off course and not finding something. A compass is a magnet, and will affect