Compass corrections — TVMDC, deviation and azimuth check

KnotWise includes three calculators for working with compass corrections. Compass Correction converts between true, magnetic and compass course using variation and deviation. Compass Check via Azimuth verifies your compass against the sun's true azimuth. Leeway calculates the angle and course correction needed for wind drift. All three run fully offline and are available on every plan.

Compass Correction (TVMDC)

Converts between the five linked values: True, Variation, Magnetic, Deviation, Compass. Enter any combination of known values; the calculator solves the chain and returns the rest.

The calculator handles the sign convention automatically. Enter each correction with its direction (East or West) — the calculator adds or subtracts as required. The chain works in both directions: from true to compass for course planning, or from compass to true for plotting bearings.

When reading variation from the chart, note the annual change. A chart printed in 2018 with «7°W (2018), decreasing 8' annually» gives 6°W approximately in 2026. KnotWise does not apply annual variation correction automatically — enter the current value.

Compass Check via Azimuth

Verifies the total compass error by comparing the sun's computed true azimuth against a bearing you observe with the compass. The calculator returns the combined variation and deviation error as a single value, with direction (high or low).

To isolate the deviation contribution, compare the total error against the variation alone — for example, a 9° W total error in an area where the chart shows 6° W variation indicates approximately 3° W deviation on the current heading.

This calculator shares its astronomical logic with the celestial cluster. See the celestial navigation help article for the full input list and a worked example.

Most accurate when the sun is low — within roughly an hour of sunrise or sunset. High sun altitudes amplify small errors in azimuth calculation.

Leeway

Calculates the leeway angle — the difference between the heading you steer and the track you actually make good — based on wind angle, wind speed, boat speed and an empirical leeway coefficient.

The output is the leeway angle in degrees and the course correction needed to compensate. Apply the correction upwind: to make a true course of 240° with 5° of leeway and wind from the north, steer 235° magnetic-converted-to-compass.

The leeway coefficient is the trickiest input. Sailboat values typically fall between 3° and 8° depending on point of sail and conditions. For the most reliable figure, observe your wake from the stern — the angle between the wake and the centreline is your actual leeway. Record this on different points of sail and conditions to build a working table for your vessel.

Shared features across the compass cluster

All three calculators include an expandable formula panel, a source note specifying the recommended reference (chart for variation, deviation card for deviation), and the standard Export PDF and Save to Logbook actions. Inputs are kept only for the current session.

Accuracy and limits

• Compass Correction: Exact arithmetic. The result is only as accurate as the variation and deviation values you enter.
• Variation: Read from the compass rose on the chart for the current epoch. Annual change matters on older charts — a chart from 2015 may now show variation 1° to 2° off the actual value.
• Deviation: Vessel-specific and heading-specific. Must be re-swung if the magnetic environment changes — new engine, ferrous items added, generator installed, autopilot drive replaced. Trust the deviation card only if it is current.
• Compass Check via Azimuth: Most accurate when the sun is low. High altitudes amplify azimuth calculation errors. Check on at least three different headings to characterise deviation across the compass rose.
• Leeway: Empirical. The coefficient varies with sail configuration, sea state and wind angle. Treat the calculator output as a planning figure; refine through observation.

Frequently asked questions

«East is least, West is best» — does it apply here?

Yes — it is the mnemonic for converting compass to true. Easterly corrections are subtracted, westerly are added. The calculator handles the sign convention automatically when you enter each value with its direction; you do not need to memorise the mnemonic to use the tool.

Can I work from a deviation card with discrete heading entries?

Yes. Most deviation cards list deviation at headings of every 15° or 30°. Read the value for the heading nearest to your compass course and enter it. For more precision, interpolate between adjacent entries.

How often should I check my compass against the sun?

At minimum once per major passage. Always after any change to the magnetic environment of the boat — new equipment, repositioned ferrous items, hull repairs near the compass. Whenever you suspect drift, take a fresh azimuth sight on a clear horizon.

What leeway coefficient should I use?

Empirical, by observation. Typical sailboat values range from 3° close-hauled in light conditions to 8° or more on a beam reach in heavy weather. Observe your wake angle from the stern on different points of sail and conditions, and build a table for your vessel. The number on the calculator is a planning figure; the wake is the truth.

Should I correct for annual change in variation automatically?

No — enter the current value yourself. KnotWise does not apply annual variation correction. Read the chart entry and adjust manually. On an old chart printed in 2015, the original 7° W variation may now be closer to 5° W depending on location. Verify against a recent chart or an online geomagnetic calculator if in doubt.