The exact value of the magnetic declination is place-dependent and varies over time, though declination is frequently given on the map itself or obtainable on-line from various sites. If the hiker has been following the correct path, the compass’ corrected (true) indicated bearing should closely correspond to the true bearing previously obtained from the map.
Compass navigation in conjunction with a map (terrain association) requires a different method. To take a map bearing or true bearing (a bearing taken in reference to true, not magnetic north) to a destination with a protractor compass, the edge of the compass is placed on the map so that it connects the current location with the desired destination (some sources recommend physically drawing a line).
The beam compass is used to scribe a circle. The radius can be adjusted by sliding the metal across the wood (beam) and locking it by turning a knob at the desired location. The threaded machine rod is similar to the bolt. The only limitation is the rigidity of the wood being used. Longer pieces tend to get floppier depending on the species of wood used. Metal can be used as an alternative but also has length limitations. Trammel points score a precise cut out line with the sharp point of the rod.
Mariners are concerned about very accurate measurements; however, casual users need not be concerned with differences between magnetic and true North. Except in areas of extreme magnetic declination variance (20 degrees or more), this is enough to protect from walking in a substantially different direction than expected over short distances, provided the terrain is fairly flat and visibility is not impaired. By carefully recording distances (time or paces) and magnetic bearings traveled, one can plot a course and return to one’s starting point using the compass alone.
A magnetic compass points to magnetic North pole, which is approximately 1,000 miles from the true geographic North Pole. A magnetic compass’s user can determine true North by finding the magnetic North and then correcting for variation and deviation. Variation is defined as the angle between the direction of true (geographic) north and the direction of the meridian between the magnetic poles.
Because the Earth’s magnetic field’s inclination and intensity vary at different latitudes, compasses are often balanced during manufacture. Most manufacturers balance their compass needles for one of five zones, ranging from zone 1, covering most of the Northern Hemisphere, to zone 5 covering Australia and the southern oceans. This balancing prevents excessive dipping of one end of the needle which can cause the compass card to stick and give false readings.
The compass is very stable in areas close to the equator, which is far from “Magnetic North”. As the compass is moved closer and closer to one of the Magnetic Poles of the Earth, the compass becomes more sensitive to crossing the magetic field lines of the Earth. At some point close to the Magnetic Pole the compass will not indicate any particular direction but will begin to drift in a non direction indicating manner. Also, the needle starts to point up or down when getting closer to the poles, due to the so-called magnetic inclination. Cheap compasses with bad bearings may get stuck due to this and therefore indicate a wrong direction.
The compass functions as an indicator to “Magetic North” because the magnetic bar at the heart of the compass aligns itself to one of the lines of the Earth’s magnetic field. Depending on where the compass is situated on the surface of the Earth the variance between geographic North or “True North” will increase the farther one is from the prime meridian of the Earth’s magnetic field. It should be noted that the geographic North Pole and the Magnetic North pole are not coincident on the surface of the Earth.
Small compasses found in clocks, mobile phones, and other electronic devices are solid-state compasses, usually built out of two or three magnetic field sensors that provide data for a microprocessor. The correct heading relative to the compass is calculated using trigonometry.
Large ships typically rely on a gyrocompass, using the magnetic compass only as a backup. Increasingly, electronic fluxgate compasses are used on smaller vessels. However compasses are still widely in use as they can be small, use simple reliable technology, are comparatively cheap, often easier to use than GPS, require no energy supply, and unlike GPS, are not affected by objects, e.g. trees, that can block the reception of electronic signals.
