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An azimuth compass is a compass that displays direction or bearing markings in terms of azimuth angles rather than any other form of bearing notation. There are two principal systems used to display direction markings on the face of a compass: quadrant bearings and azimuth bearings. Both systems provide exactly the same directional information but in a different format. People who have used a compass have most likely used an azimuth compass.
The quadrant bearings system marks both the North and South positions as zero degrees. Direction is then measured in degrees using both North and South as the base reference points. For example, starting at North and moving clockwise 35° results in a quadrant bearing N35°E. Alternatively, starting at East and moving counterclockwise 55° results in the same bearing N35°E. In this system, North or South will always be the first notation in the bearing.
The most widely used compass markings are those of the azimuth bearings system. In this system, the only position marked as zero is North. Moving clockwise, degrees are added through the full 360° of a complete circle. In this system, East is marked as 90°, South as 180°, and West as 270°.
The following table further clarifies the two systems. The table provides eight separate direction headings each expressed first as a quadrant bearing and then as an azimuth bearing.
Azimuth bearings are based on the concept of an azimuth angle, also referred to simply as an azimuth. An azimuth is defined, from any given observation point, as the angle between an object or point and a reference line, usually to true North, moving away from that reference line in a clockwise direction on a horizontal plane. For example, standing on a ship with the ocean as the horizontal reference plane, the angle between true North and, say, a lighthouse is the azimuth for that lighthouse relative to the ship, that angle being measured by moving clockwise away from the line to true North.
Similarly, if that ship is traveling along a bearing S10°W, the navigator on that ship imagines a line drawn along that bearing intersecting with a line drawn from the ship to true north. The 190° angle formed by those two lines is the azimuth that defines the direction in which the ship is traveling relative to true north. An azimuth compass on that ship would indicate a bearing of 190°.
The largest azimuth angle that may ever be measured is 359 degrees 59 minutes 59 seconds, often written as 359° 59' 59", this being slightly less than the 360° defined by a full circle. Technically, based on the azimuth bearings system, it is incorrect for any point on the compass to be marked as both 0° and 360°. This is why an azimuth compass, by convention, marks N as 0°.
The concept of an azimuth angle is used frequently in a wide range of practical applications. These include land navigation, celestial navigation, mapping and artillery. The key feature distinguishing an azimuth compass from any other compass is the direction markings shown on the face of the compass rather than any difference in the technical workings of the compass.
Interesting article. I’ve always wondered why compasses are marked differently. Your piece helps to put it into context. Azimuth, rather than quadrant bearing, markings are easiest for me to work with. Thanks again for the note.