The term compass can refer to a navigational tool which points in a fixed direction, allowing travelers to find their way by inferring other directions and paths through the process of orienteering. When used alone the term usually refers to a magnetic compass. The plural compasses can also refer to a device used for drawing circles.
Magnetic compasses rely upon magnetized pieces of metal, typically floating in liquid to ensure low friction. These align themselves with the Earth's magnetic field lines which point at the Earth's magnetic north pole. Magnetic field lines vary from true north (pointing to the geographic north pole) and thus must be adjusted according to information usually found on a map or chart. Maps and charts have a compass rose printed with the deviation from true north noted for the area of coverage of the map. Deviation often occurs due to large mineral deposits which occur in the area of interest.
Deviation from true north can also occur due to the presence of nearby large metal objects, such as ships at sea. At sea, this additional magnetic deviation is tracked, logged, and factored in to reading a magnetic compass in order to determine true north. In an aircraft the aircraft's movement can affect a compass, causing it to give inaccurate readings during turns and other maneuvers.
Magnetic compasses were first invented as a device for divination as early as the Chinese Han Dynasty (since about 206 BC). Magnetic compasses were in early use by Portuguese explorers in the Age of Exploration.
Compasses are one of the two most essential pieces of navigation gear that one should carry, the other being a map. GPS devices and cell phones with Google Maps may breakdown or run out of batteries during an emergency, disaster or a Post Apocalyptic World - SHTF - TEOTWAWKI event. Also the sun isn't always shining, and it takes a great deal of time and effort to learn to stellar navigation, however a map and compass will continue to work until your break or lose them.
Few things are more important than knowing where you are and where you're going and the compass helps you to do both. In selecting a compass there many types with many different features to consider, some of them are novelties while others are tremendous conveniences. There are several basic types of compass, and some compasses fall under more than one type.
Types of Compass
A gyrocompass, also known as a direction indicator, is another instrument designed to indicate north. They are mainly used in aircraft, where the change in altitude of an aircraft can affect the direction of a normal magnetic compass. The principle of a gyrocompass relies upon a rapidly spinning wheel (usually run electrically) which maintains its original orientation. Due to the law of conservation of angular momentum, a gyrocompass can be set to point to true north when it is initialized, and it will continue to point to true north. Gyrocompasses will drift over time, and therefore need to be resynchronized with a magnetic compass at regular intervals.
Baseplate compass are generally clear with various markings on them, they are designed to be easier to navigate with a map. They are often very feature rich and come at a variety of price points.
Sighting Compasses generally have either a mirror or sighting whole on perpendicular folding part of the compass. They allow you to look at both the compass and a landmark and line them up for better navigation.
The Transit is the "grand daddy" of compasses, usually including many features, however these are more of a miner or engineer's tool. They are extremely expensive, but offer many abilities other compasses do not.
Magnetic North is not in the same location as true North and depending on where you are in the world the difference between a magnetic North reading and where truth North is could vary by a couple of degrees or 30. A Declination scale allows you to calculate or adjust for this difference by knowing approximately where you are in the world and what the deviation is. If you have a fixed declination scale you will have to do the math, but if you have an adjustable deviation scale you can set it and forget it until you are in a location far away.
A compass that floats could be a life saver in some situations.
A lanyard attachment is a common and highly recommended feature, this will help you to avoid losing your compass.
If you have to navigate at night, glow in the dark compass point will be extremely helpful.
- Merrill, Ronald T. (1983). The Earth's magnetic field: Its history, origin and planetary perspective, 2nd printing, San Francisco: Academic press. ISBN 0-12-491242-7.
- Admiralty, Great Britain (1915) Admiralty manual of navigation, 1914, Chapter XXV: "The Magnetic Compass (continued): the analysis and correction of the deviation", London : HMSO, 525 p.
- Lane, Frederic C. (1963) "The Economic Meaning of the Invention of the Compass", The American Historical Review, 68 (3: April), p. 605–617
- Gurney, Alan (2004) Compass: A Story of Exploration and Innovation, London : Norton, ISBN 0-393-32713-2
- Johnson, G. Mark, The Ultimate Desert Handbook, 1st Ed., Camden, Maine: McGraw-Hill (2003), ISBN 0-07-139303-X
- David Buisseret, ed., Monarchs, Ministers and Maps: The Emergence of Cartography as a Tool of Government in Early Modern Europe. Chicago: University of Chicago Press, 1992, ISBN 0-226-07987-2
- The Complete Idiot's Guide to Geocaching by Jack W. Peters (ISBN 1-59257-235-9)
- Geocaching For Dummies by Joel McNamara (ISBN 978-0-7645-7571-6)
-  Handbook of Magnetic Compass Adjustment.
- How to Make a Compass Audio slideshow from the National High Magnetic Field Laboratory
- Paul J. Gans, The Medieval Technology Pages: Compass
- The Gear Junkie - review of two orienteering thumb compasses
- COMPASSIPEDIA, the great virtual Compass Museum gives comprehensive information about all sorts of compasses and how to use them.
- Geography fieldwork
- International Orienteering Federation
- YouTube.com: Orienteering, USOF style