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Figure 10.7. Azimuths and bearings

Angles can be measured with different instruments: a magnetic
compass, a theodolite and others. Unfortunately, the Earth's magnetic

field does not yield the most reliable measurements. The magnetic
poles are not aligned with the planet's axis of rotation (an effect called
magnetic declination), and they tend to change location over time.
Local magnetic anomalies caused by magnetized rocks in the Earth's

crust and other geomagnetic fields make matters worse. That`s why a
magnetic compass isn`t the best one for this purpose and surveyors
have to use more precise instruments: theodolites or total stations.

Theodolites are precision instruments used for measuring angles,
electronic theodolites read and display angles automatically. Optical
theodolites need to be read manually. Both are usually classified
according to the smallest reading that the instrument displays, this

varies from 1’ to 0.1’.
Total stations are also precision instruments that can measure
angles and distances. These are classified according to their angle and

distance measuring capability.
To achieve the desired accuracy, surveyors must overcome errors
caused by faulty instrument calibration; wind, temperature, and soft

ground; and human errors, including misplacing the instrument and
misreading the measurement wheels. In practice, surveyors produce
accurate data by taking repeated measurements and averaging the

results.
Most nowadays land surveyors rely on transits (or their more
modern equivalents, called theodolites) to measure angles. A transit
consists of a telescope for siting distant target objects, two

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