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 Angular Measurement
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Surveying and navigation often rely on the measurement of two phenomena in order to determine position, those of distance and direction or bearing. The 'Angular Measurement' section will introduce the concept of bearings and cover the instrumentation that has been developed over the centuries to facilitate the determination of relative and absolute ‘bearing’. In order to start, we will look at the definition of some terms specific to the determination of direction.
Definitions
Directions
Simply that, a direction (over there)
Bearings
A direction relative to a datum
Whole-circle bearings
The direction of survey lines is generally expressed as an angle measured from a reference meridian, generally north, commencing from 0 degrees (0°) and increasing clockwise to 360 degrees (359°59'60"). Bearings are never expressed as "North, X degrees East".
Angles
The arithmetic difference between two directions or bearings.
Reference meridians
True north (through the geographic poles about which the Earth rotates)
Magnetic north - through which lines of magnetic flux pass
Grid north - mathematically determined value
An arbitrary meridian - one adopted for a particular project
Magnetic meridian - the direction of the earth's magnetic lines of force. This varies with date, time and locality. Magnetic declination
The angle between the magnetic and true meridians.
Angle measurement is a fundamental part of surveying field observations, as the combination of a direction and a distance gives a polar vector to a point and hence a unique location of that point in space. The instruments that have been developed to facilitate angle (or direction) measurement are the magnetic compass, the sextant and the theodolite.
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