Compass deviation is determined by precisely comparing your vessel's magnetic compass reading to a known magnetic direction, after accounting for local magnetic variation. This process helps create a "deviation card," which allows navigators to correct their compass readings for accurate steering and position fixing.
Understanding Compass Deviation
Your vessel's magnetic compass points towards magnetic north, but local magnetic fields created by the ship's structure, electrical systems, and cargo can cause the compass needle to deflect. This deflection is called compass deviation. Unlike magnetic variation, which is a geographical phenomenon, deviation is specific to each vessel and can change with its heading, magnetic cargo, or the activation of certain electrical equipment.
Correcting for deviation is crucial for accurate navigation, ensuring that the course steered by compass aligns with the desired magnetic or true course.
Methods for Determining Compass Deviation
There are several practical methods for finding compass deviation:
1. Using a Transit Line
One effective way to find deviation is by using a known transit line. A transit line is formed by two fixed objects that appear in line from a specific viewpoint, providing a precise bearing.
- Identify a Transit Line: Locate two distinct objects ashore that, when viewed one behind the other, form a clear line. Determine the true bearing of this transit line from a chart or survey.
- Calculate Magnetic Bearing: Adjust the true bearing of the transit line by applying the local magnetic variation. This gives you the magnetic bearing of the transit, which represents the actual magnetic direction of the line.
- Formula: Magnetic Bearing = True Bearing ± Magnetic Variation (add if variation is Easterly, subtract if Westerly).
- Align and Observe: Steer your vessel directly along this transit line. While on the transit, ensure your ship's head is pointing precisely down the line. Note the reading on your magnetic compass, which is your compass heading.
- Calculate Deviation: The difference between the calculated magnetic bearing of the transit (your actual magnetic heading) and your observed compass heading is the deviation for that particular heading.
- Formula: Deviation = Magnetic Heading (of transit) - Compass Heading
- If the result is positive, the deviation is Easterly (E).
- If the result is negative, the deviation is Westerly (W).
- Formula: Deviation = Magnetic Heading (of transit) - Compass Heading
- Repeat: To build a comprehensive deviation card, repeat this process on different headings (e.g., every 15 or 30 degrees) around the compass rose.
2. Swinging the Compass
Swinging the compass is a systematic method performed in an open area with minimal magnetic interference:
- Choose a Reference: This method requires a known magnetic bearing, such as a distant object whose magnetic bearing has been predetermined, or by comparing it to a precisely calibrated gyro compass if available.
- Turn the Vessel: Steer the vessel slowly through a full 360-degree circle, pausing at regular intervals (e.g., every 15, 20, or 30 degrees of the magnetic compass).
- Take Bearings: At each interval, simultaneously note the ship's magnetic compass heading and the bearing of your chosen reference object using both the magnetic compass and the known accurate magnetic bearing (or gyro bearing corrected to magnetic).
- Calculate Deviation: For each heading, compare the compass bearing of the reference object with its known magnetic bearing. The difference is the deviation for that particular heading.
3. Using Celestial Bearings
For vessels at sea, celestial bodies (like the sun, moon, or stars) can provide reliable true bearings:
- Calculate True Bearing: Use navigational almanacs or software to calculate the precise true bearing of a celestial body at a specific time and your vessel's position.
- Observe Compass Bearing: At that exact moment, take a bearing to the celestial body using your ship's magnetic compass.
- Convert and Compare: Apply local magnetic variation to the calculated true bearing to get the magnetic bearing. Then, compare this magnetic bearing with your observed compass bearing to find the deviation. This method is often used for daily checks at sunrise or sunset.
4. Comparing with a Gyro Compass
If your vessel is equipped with a properly functioning gyro compass, it offers a straightforward comparison:
- Simultaneous Readings: Steer the vessel on various headings and simultaneously record the readings from both the magnetic compass and the gyro compass.
- Apply Gyro Error: Correct the gyro compass readings for any known gyro error to obtain accurate true headings.
- Apply Variation: Convert the true headings from the gyro to magnetic headings by applying local magnetic variation.
- Calculate Deviation: Compare the corrected magnetic headings (from gyro) with the magnetic compass readings to determine the deviation for each heading.
Creating a Deviation Card
After determining deviation for various headings, the data is compiled into a deviation card or table. This card is displayed near the compass for easy reference.
Here's an example of a simple deviation card format:
| Compass Heading | Deviation (E/W) |
|---|---|
| N (000°) | 3° E |
| NE (045°) | 2° E |
| E (090°) | 1° W |
| SE (135°) | 3° W |
| S (180°) | 2° W |
| SW (225°) | 1° E |
| W (270°) | 3° E |
| NW (315°) | 4° E |
Note: Deviation values are interpolated for intermediate headings.
Factors Affecting Deviation
Several factors can influence a vessel's compass deviation:
- Permanent Magnetism: Inherent magnetism in the ship's steel structure.
- Induced Magnetism: Magnetism temporarily induced in soft iron by the Earth's magnetic field, which changes as the vessel's heading changes.
- Electrical Currents: Current-carrying wires, especially near the compass.
- Magnetic Cargo: Steel, iron, or other magnetic materials carried as cargo.
- Nearby Electronic Devices: Unshielded radios, speakers, or other electronic equipment.
- Structural Changes: Major alterations or repairs to the vessel.
- Location: While variation changes with location, deviation is specific to the vessel and its magnetic environment. However, the induced magnetism component of deviation can slightly vary with latitude.
When to Check Deviation
It is crucial to check and correct compass deviation regularly:
- After new construction or major repairs: Especially if new magnetic materials are introduced.
- After dry-docking: The Earth's magnetic field can re-magnetize the hull.
- After a prolonged stay on one heading: Particularly in port.
- After carrying magnetic cargo.
- If the vessel experiences a hard grounding or lightning strike.
- At least annually, or as required by regulations.
- Whenever there's a significant discrepancy between compass readings and other navigational aids.
Regularly checking and correcting for compass deviation ensures safe and accurate navigation, aligning your vessel's course with the intended path.
