Quick Answer
Compasses near large metal structures may not provide reliable readings due to magnetic interference, which can deviate the compass needle from its intended direction.
Magnetic Deviation and Compass Error
Compasses rely on the Earth’s magnetic field to determine direction. However, large metal structures like buildings, bridges, or pipelines can generate strong magnetic fields that interfere with the compass’s accuracy. This phenomenon is known as magnetic deviation or compass error.
Quantifying the Problem
The extent of magnetic deviation depends on the proximity and strength of the metal structure. As a general rule, if you’re within 100 feet (30 meters) of a large metal structure, expect a compass reading error of around 1-5 degrees. However, if you’re within 50 feet (15 meters), the error can be as high as 10-20 degrees.
Mitigating Compass Error
To compensate for magnetic deviation, use a compass with a declination adjustment or a rotating bezel. Set the declination adjustment to match the local magnetic variation (also known as declination) for your area. Alternatively, use a compass with a rotating bezel, and rotate it to match the local magnetic variation. If possible, find a safe location away from the metal structure to take an accurate reading. In extreme cases, consider using a GPS device or a sighting technique like the “shadow stick” method to determine direction.
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