Declination (magnetic)
Overview
Magnetic declination is the angular difference between true north and magnetic north at a specific location. It affects how a compass bearing relates to map directions.
Key points
- Varies by geographic location and changes slowly over time.
- Expressed in degrees east or west of true north.
- Must be accounted for when converting between map bearings and compass bearings.
- Some compasses allow built-in declination adjustment.
- Digital navigation tools may display bearings relative to true or magnetic north.
Details
Magnetic declination arises because the Earth’s magnetic field, to which a compass needle aligns, does not perfectly match the geographic poles used by maps. The angle between magnetic north and true north differs from place to place and gradually changes as the magnetic field shifts.
In practical terms, hikers using a traditional magnetic compass and a map may need to add or subtract the local declination value when converting between compass bearings and map bearings. This prevents navigation errors that accumulate over distance, particularly in open terrain where precise headings matter.
Some modern compasses allow users to set a fixed declination offset so that the needle aligns with true north directly when oriented to map north. Digital devices often give users the option to work with either true or magnetic north in their settings.
For many on-trail hikers in well-marked corridors, fine bearing work is less frequent, but understanding declination remains part of basic navigation education and becomes more important in off-trail or low-visibility scenarios.
Related topics
- contour-interval
- contour-line
- introductory-navigation-skills-for-beginners
- navigation-and-route-finding-overview
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