Compass
A compass is a device which has the basic function of taking a bearing.
The magnetic compass
A magnetic compass is a simple device that has a small magnet mounted on a near frictionless spindle or needle. Typically the body of a compass body is constructed of metal or strong plastic. A compass such as a “ Brunton” compass has a built in sighting clinometer and bull’s-eye level.
Using a compass
When a compass is held level, its needle will seek magnetic North from your location. This is because of the “opposites attract” effect of the small magnet on the needle as it is affected by the Earth’s magnetic field. The ’North-seeking’ end of a compass is generally indicated in white although colours may vary, or it may simply have an “N” for North. A compass bearing is the geographic direction from one point to another and is generally known as the cardinal direction - which is noted in writing as a degree from this cardinal direction using first the capital letter designating North (N), South (S), East (E) or West (W), next the degree bearing and the next cardinal direction nearest the needle bearing as in N 20 E (read as North 20 degrees East) [1]. An alternative way of stating this same bearing is through the use of the azimuth scale, which uses the face of the compass as a 360 degree circle with readings from 0 degrees to 359 degrees. Thus the bearing N 20 E would be stated as 20 in an azimuth scale and a reading of N 20 W would be read as 340 in an azimuth scale[1]. When sighting with a compass, it is critical to hold the compass level, centring the bubble in the bullseye level [1]. Hold the compass at waist level. For compasses with no mirror rotate the bezel until North is in line with the direction of the travel arrow. Rotate the levelled compass on a vertical axis until the direction of travel arrow is pointing along the desired line of bearing. Check the compass level and read the bearing indicated by the North-seeking end of the needle (the North-seeking end of a compass is typically white although this may vary from compass to compass) [1]. The bearing indicated is the bearing sighted along the line of the direction of travel arrow. In more sophisticated compasses that are equipped with a mirror, level the compass as above and adjust the mirror in the lid of the compass until the sighting tip and point sighted both appear in the mirror. Rotate the compass on a vertical axis until the sighting tip and the point sighted meet with the axial line of the mirror. The bearing indicated by the North-seeking tip is the bearing to the point sighted.
Electronic compasses
There are also a large variety of electronic compasses including compasses contained within other devices such as a GPS or mobile telephone. These compasses are interpreted or read in much the same manner as a magnetic compass (although they may have a digital readout with only letters and numbers) but operate on a different principle. Electronic compasses determine North my measuring the relative strength of magnetic fields that pass through two coils of wire. Using software and electronics, these sensors can deduce the direction of the earth’s magnetic field.
Adjusting bearing for magnetic declination
Declination is the difference between magnetic North and true North[2]. Magnetic declination at ones location may be noted from the legend of almost any topographic map of an area and would usually be indicated in both text and with an illustration showing a pair of directional arrows forming a measurable angle[2]. As local magnetic declination changes or ‘drifts’ over time, estimated change is usually also given. For accurate readings it is therefore wise to use as recent a map as is possible. A typical declination reading in the legend of a map might be stated as “Mean magnetic declination 20 00’ West of True North (Jan. 1974). Mean annual change 3’ Eastwards (1970 – 1975).” Declination is adjusted for by adjusting the graduated compass circle the amount and direction of the local declination. In most compasses you simply turn the graduated circle on the outer ring of the compass. In the Brunton-type compass, declination is adjusted by means of a screw on the side of the compass. In the case of the example given above, for a reading taken in 1974 the graduated circle would be moved until the index pin located at the base of the sighting arm points to 20 on the side of the graduated circle marked with a W.
Magnetic effect of metal
Never take a reading near a metallic object or an object containing large amounts of metal<ref name="Berger1">. This includes items such as your belt-buckle, pocket knife, cars or even reinforced concrete. Steel and iron items such as these have there own magnetic fields and can cause magnetic irregularities or anomalies and your compass to misread. Magnetic anomalies may also be caused by certain features in nature, iron-bearing rocks or rocks containing magnetic material for example. Natural magnetic anomalies may be indicated when a foresight and back-sight on widely separated sighted points do not agree.
References
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