How To Correct Magnetic Compass Bearings

Summary

This educational video script delves into the importance of correctly applying compass readings to nautical charts. It explains the concept of 'variation'—the difference between magnetic and true north—and how to find it on a nautical chart. The script also addresses 'deviation,' the local magnetic field's effect on compass readings, and provides methods to calculate true bearings from compass bearings and vice versa. It simplifies the process with diagrams, mnemonics, and practical examples, ensuring viewers can navigate accurately even with a magnetic compass.

Takeaways

• 🧭 The importance of using compass readings correctly on a nautical chart is due to the difference between the geographic and magnetic north poles.
• 🌐 Geographic north is the reference point for bearings on a chart, but magnetic compasses point to the magnetic north, which can vary in location.
• 📊 Variation is the difference between true north and magnetic north and is crucial for accurate navigation; it can be found on nautical charts and electronic navigational aids.
• 📈 Variation changes over time, with an example given of an annual increase of six minutes, which navigators must account for when calculating current variation.
• 🔍 Deviation refers to local changes in the magnetic field affecting compass readings, often due to the metal in a ship's hull or nearby objects.
• 📝 Deviation is measured using a deviation card, which provides the compass's deviation at various headings, essential for accurate navigation.
• 🔄 To convert a true bearing to a magnetic bearing, apply the variation; to convert a magnetic bearing to a true bearing, subtract the variation.
• 📉 When applying deviation, if it's east, the compass reading is less than the magnetic bearing; if west, the compass reading is more.
• 🔢 For converting chart bearings to compass bearings, add the variation if it's west, and subtract if it's east; for converting compass bearings to chart bearings, do the opposite.
• 📚 Mnemonics like 'deviation east, compass least' and 'variation west, magnetic best' can help remember how to apply these corrections, though diagrams are also a clear method.
• 🔗 Additional resources and articles are available to further understand the application of variation and deviation in navigation.

Q & A

Why is it necessary to use compass readings in conjunction with a nautical chart?

Compass readings are essential because they provide bearings relative to the magnetic north pole, which can differ from the geographic north pole. Aligning these readings with a nautical chart, which references the geographic north, is crucial for accurate navigation, especially over long distances where even small errors can result in significant deviations.

What is the difference between the geographic north pole and the magnetic north pole?

The geographic north pole is a fixed point on Earth, around which all latitude and longitude measurements are based. The magnetic north pole, however, is where the Earth's magnetic field points and can vary, sometimes significantly, from the geographic north pole.

What is 'variation' in the context of navigation?

Variation is the angular difference between magnetic north and true north. It is used to correct the compass readings so that they align with the true north indicated on nautical charts.

Where can you find the variation on a nautical chart?

Variation can be found on a nautical chart within the compass rose, where an arrow points towards the magnetic north pole, and the difference between it and the true north is indicated.

How does the annual increase of variation affect navigation?

The annual increase of variation must be accounted for to ensure that the compass readings are accurate for the current year. For example, if the variation was 4 degrees 30 minutes west in 2015 and there's an annual increase of six minutes, by 2022 the variation would have increased, and this must be considered when navigating.

What is deviation in the context of compass readings?

Deviation refers to the local changes in the magnetic field that affect the compass's指向, causing it to not align perfectly with the Earth's magnetic field. This can be influenced by the metal in a ship's hull, cargo, or other magnetic materials nearby.

How can you determine the deviation of a compass?

Deviation can be determined through a process known as a compass swing, which involves taking readings at different headings and comparing them to a known reference to identify any discrepancies.

What is the acronym 'CDMVT' used for in navigation?

CDMVT is a mnemonic to remember the order of corrections when converting a compass bearing to a true bearing, standing for Compass, Deviation, Magnetic, Variation, True.

Can you provide an example of how to apply variation to a true bearing?

If you have a true bearing of 072 degrees and the variation is 5 degrees west, you would add the variation to the true bearing, resulting in a magnetic bearing of 077 degrees.

How do you convert a magnetic bearing to a compass bearing considering deviation?

If you have a magnetic bearing and know the deviation, you adjust the magnetic bearing by the amount of deviation. For example, if the magnetic bearing is 077 degrees and the deviation is 1 degree east, you would subtract 1 degree from the magnetic bearing to get a compass bearing of 076 degrees.

What is the process for converting a compass bearing to a true bearing for plotting on a nautical chart?

To convert a compass bearing to a true bearing, first correct for deviation to get a magnetic bearing, then apply the variation to get the true bearing. This can be visualized using a diagram to understand the angular relationships between compass, magnetic, and true north.