Variation in Coastal Navigation

Variation in coastal navigation refers to the angular difference between true north (geographic north) and magnetic north at a specific location. This variation, also known as magnetic variation or magnetic declination, is a crucial factor to consider when using a magnetic compass for navigation.

Here’s how variation impacts coastal navigation:

Magnetic Compass: A magnetic compass points towards magnetic north, not true north. Magnetic north is the direction to which the magnetic needle of the compass aligns due to the Earth’s magnetic field. However, true north, also known as geographic north, is the direction to the North Pole.

Variation Values: The magnetic variation can vary significantly depending on your location on the Earth’s surface. It is expressed in degrees east or west, and its value changes over time due to changes in the Earth’s magnetic field. In coastal navigation, variation values are typically available on nautical charts or in navigation publications.

Accounting for Variation: To navigate accurately, mariners need to account for the local magnetic variation when using a magnetic compass. This is done by applying the variation value to convert magnetic bearings or courses to true bearings or courses.

East Variation: If the magnetic needle points east of true north (geographic north), the variation is considered east, and you need to add it to a magnetic bearing or course to get the true bearing or course.

West Variation: If the magnetic needle points west of true north, the variation is considered west, and you need to subtract it from a magnetic bearing or course to get the true bearing or course.

Practical Example: For example, if the magnetic variation in a coastal area is 5 degrees east, and your magnetic compass indicates a bearing of 60 degrees, the true bearing would be 65 degrees (60 degrees + 5 degrees east variation).

Updating Variation: Mariners must regularly update the variation value they use for navigation, as it changes over time. This information can be obtained from charts, navigation publications, or online sources.

Chartwork: Nautical charts typically include lines of equal magnetic variation (isogonic lines) to help mariners determine the variation for a specific location. These lines indicate the angular difference between true north and magnetic north at various points on the chart.

In coastal navigation, accurately compensating for magnetic variation is essential to ensure that a vessel follows the intended course and avoids navigational errors.

Neglecting variation can lead to significant deviations from the desired track, potentially resulting in navigation hazards or straying off course. Therefore, understanding and applying variation correctly is a fundamental aspect of safe and accurate coastal navigation.