The compass The compass is used to determine the direction of celestial bodies. Compasses come in two main types - gyroscopic and magnetic. Gyro compasses, if set correctly, always point to true north. Magnetic compasses almost never point to true north as the magnetic pole is nowhere near the true pole. A magnetic compass needs a number of corrections for it to give true north. A mnemonic to remember these is Can Dead Men Vote Twice?
C = reading of the compass Deviation is that error caused by the situation of the compass and metals in its vicinity. A well adjusted compass should have a deviation of less than 3 degrees. To determine the deviation the object on which the compass is housed is "swung" and a deviation card drawn up showing the deviation for each degree. This card should be available near the compass.
 Deviation Card Variation is that difference between true North and magnetic North and can be obtained from the chart of the area. As variation changes with time, adjustments must be made according to date of the chart. Local variation and annual change can be calculated at this page. |