Chart Scale Distance Calculator

Enter your true course, magnetic variation, compass deviation, and distance to calculate your compass heading, magnetic heading, total correction, and radar range reference.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter the True Course
Input the course measured from true north on your nautical chart, typically obtained from route planning software or a chart plotter.
2
Specify Magnetic Variation
Provide the angle between true north and magnetic north for your current location. This value changes geographically and over time; use negative for West variation (e.g., -7 for 7°W).
3
Input Compass Deviation
Enter the error caused by your vessel's magnetic fields affecting the compass. This is specific to your boat and changes with heading, often found in a deviation card. Use negative for West deviation.
4
Set the Passage Distance
Indicate the distance of the specific leg or passage you are planning, in nautical miles (NM).
5
Review Your Results
The calculator will display your required Compass Heading, Magnetic Heading, and other critical navigation data.

Example Calculation

A sailor is planning a coastal passage and needs to determine their compass heading from a chart's true course, accounting for local magnetic variation and their vessel's deviation.

True Course (°)

120

Magnetic Variation (°)

-7

Compass Deviation (°)

Distance (NM)

Results

125.0°

Tips

Verify Variation Annually

Magnetic variation changes slightly each year. Always check the latest nautical chart or a reliable digital source for the most current variation for your area, especially for long passages.

Create a Deviation Card

To ensure accurate compass headings, swing your compass and create a deviation card for your vessel. This process identifies how your boat's electronics and metal affect the compass at various headings.

Understand Compass Error

A significant 'Total Compass Correction' (e.g., over 10°) indicates a large difference between true and compass headings. Always be aware of this total correction, as it directly impacts your steered course accuracy.

Converting Nautical Courses for Safe Passage Planning

Navigating open waters or coastal channels requires precise understanding of direction, a skill heavily reliant on accurate course calculations. The Chart Scale Distance Calculator assists mariners in converting a true course, derived from nautical charts, into the magnetic and compass headings necessary for steering a vessel. This conversion is critical for safety and efficiency, especially when navigating through busy shipping lanes or complex coastlines, where an error of even a few degrees can lead to being off course by miles over a 20-nautical-mile passage.

Accurate course correction is paramount for several reasons beyond simply reaching a destination. It ensures safety by preventing collisions with other vessels or navigational hazards, optimizes fuel consumption by minimizing deviations, and guarantees compliance with maritime regulations. Neglecting to properly account for magnetic variation and compass deviation can result in a vessel unknowingly drifting off its intended track, potentially into dangerous shoals or restricted zones. For example, in regions with significant magnetic variation like the North Atlantic, values can exceed 15° West, making a direct true-to-compass conversion without correction highly risky.

The Navigational Math Behind Course Corrections

The process of converting a true course to a compass heading involves a series of adjustments. First, magnetic variation is applied to the true course to find the magnetic heading. Then, compass deviation is applied to the magnetic heading to determine the compass heading. The formula is:

magnetic heading = true course - magnetic variation
compass heading = magnetic heading - compass deviation

Where magnetic variation is negative for West variation and positive for East variation, and similarly for compass deviation. The calculator also estimates a radar range reference, which is derived from the square root of the distance. For instance, a 22 NM passage might offer a radar range reference of approximately 5.77 NM, useful for anticipating contact with targets.

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Practical Application: Charting a Course from True to Compass

Imagine a pleasure craft captain planning to sail a leg with a true course of 120° on their chart. For their current location, the magnetic variation is 7° West (input as -7°), and their vessel's deviation card shows a compass deviation of 2° East (input as +2°) on a 120° magnetic heading. The leg is 22 nautical miles.

Calculate Magnetic Heading: magnetic heading = 120° (True Course) - (-7°) (West Variation) magnetic heading = 120° + 7° = 127°
Calculate Compass Heading: compass heading = 127° (Magnetic Heading) - 2° (East Deviation) compass heading = 125°

The captain must steer 125° on their magnetic compass to follow the intended 120° true course. The calculator also provides a radar range reference of approximately 5.77 NM, suggesting effective target detection within about 6 nautical miles for this passage.

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Understanding Nautical Course Corrections

In nautical navigation, the accuracy of course corrections is paramount for safe and efficient travel. The International Hydrographic Organization (IHO) sets global standards for charting and hydrography, ensuring consistent data for mariners. Magnetic variation, the difference between true north and magnetic north, can range from near 0° in some equatorial regions to over 30° in polar areas, and typically changes by a few minutes of arc annually. For instance, in the Gulf of Mexico, variation might be 5° East, while off the coast of Maine, it could be 15° West. Compass deviation, caused by a vessel's specific magnetic interference, is unique to each ship and can vary from 0° to 10° or more, depending on the heading. Professional navigators routinely cross-reference their compass with known bearings or GPS readings to account for these dynamic forces.

The art of nautical navigation, particularly the calculation of headings, has a rich history rooted in practical observation and scientific advancement. Early mariners relied on celestial navigation and crude magnetic compasses, adjusting for known variations. The concept of magnetic variation was understood by explorers like Christopher Columbus, who noted its change across the Atlantic in the late 15th century. However, it wasn't until the 18th century that systematic methods for correcting compass deviation were developed. Pioneering work by figures such as Matthew Flinders and George Biddell Airy in the 19th century led to the formalization of deviation tables and compass adjustment techniques. These mathematical models and empirical corrections, refined over centuries, allowed for increasingly accurate passage planning and safer global exploration, underpinning the precise navigation systems still in use today alongside modern electronics.

Frequently Asked Questions

What is the difference between True Course, Magnetic Heading, and Compass Heading?

True Course is the direction of travel relative to geographic North, depicted on nautical charts. Magnetic Heading is the direction relative to magnetic North, which varies from true North due to magnetic variation. Compass Heading is the direction shown on your ship's compass, which is further affected by the vessel's magnetic deviation.

How does magnetic variation impact navigation?

Magnetic variation is the angular difference between true North and magnetic North, caused by the Earth's magnetic field. It is crucial for navigation because it dictates how you convert true courses from a chart into magnetic headings for a magnetic compass. Ignoring variation can lead to significant navigational errors, especially over long distances.

What is compass deviation and why is it important?

Compass deviation is the error induced in a magnetic compass by the local magnetic fields of the vessel itself, such as electrical equipment, engine, or metallic structures. It varies with the ship's heading. Correcting for deviation is vital for accurate navigation, ensuring that the course steered on the compass aligns with the desired magnetic heading.

How is radar range reference calculated?

The radar range reference in this calculator provides an estimated maximum line-of-sight radar detection range for an object at sea level, based on the square root of the distance in nautical miles multiplied by a constant (approximately 1.23). This is a simplified model often used for quick planning to understand potential target detection distances in clear conditions.