True vs Compass Observation App

The wind you feel on a moving boat isn't the real wind. True-Wind Calculator turns the apparent wind you measure into the true wind you need — in two taps, no wind instruments required. In use since 2013, completely rebuilt in 2026. HOW IT WORKS Enter boat speed, apparent wind speed, apparent wind angle and heading. The app does exact vector math — no lookup tables, no approximations — and gives you true wind speed and true wind direction instantly. Boat speed and heading can be filled automatically from GPS and compass with one tap. FREE • True wind speed, direction and angle — unlimited calculations • One-tap GPS fill for boat speed and heading • The formula explained, so you can trust the numbers • Works completely offline — no account, no ads, no tracking PRO (one-time purchase, no subscription) • Performance — live VMG, target boat speed, % of target VMG and point of sail, driven by your boat's polar • Tactics — tack preview, windward-mark laylines, and whether you can lay the mark • Boat polars — pick a preset or build your own in the editor • Compass rose view — heading, apparent and true wind on one dial • Calculation history — every result logged with its inputs • All speed units — knots, m/s, km/h, mph Bought the app back when it was paid? Pro is already yours — it unlocks automatically. PRIVACY Everything stays on your device. Location is read only to fill the input fields and never leaves your phone. No analytics, no third-party SDKs.

The compass check and celestial navigation toolkit for deck officers, yachtsmen and navigation students. Everything is calculated on your phone, offline, with no account, no ads and no data collection. PHONE COMPASS • Your heading, magnetic or true, with the Sun, the Moon and your chosen body marked on the dial • Find your phone's own compass error by aligning it to the Sun or the Moon; AR Sky uses the same correction COMPASS & GYRO ERROR • Gyro and standard compass error from any body's true bearing, with variation (World Magnetic Model) and deviation worked out for you • SOLAS gyro-error limit check for your latitude • Amplitude worksheet: compass error at sunrise or sunset, with the exact bearing for the Sun's centre on the visible horizon (dip, refraction and parallax applied) as well as the classic formula • Deviation cards with a fitted deviation curve and a PDF report SIGHT REDUCTION & FIX • Enter your sextant altitude in degrees and minutes as the sextant reads it • Index error, dip, refraction (with air temperature and pressure), semi-diameter and parallax, including the Moon's augmentation • Intercept and azimuth (Marcq St-Hilaire), drawn on a plotting sheet • Show calculation: every step written out with its formula, then with your sight's numbers • Fix from two or more saved sights by least squares, including running fixes with course and speed, plotted with every line of position • Meridian Passage: the time of noon for your DR, course and speed, and your latitude from the noon sight, every step shown; a morning sun line run up to noon gives the noon position • A countdown to meridian passage on your chronometer, in UTC or ship's time: it ticks faster over the last minute and sounds a tone at the passage • Latitude by Polaris at any time of the night, worked out exactly for the moment of the sight (northern hemisphere) • Ship's time: set the zone your ship keeps, and times are shown on the ship's clock • Twilight Stars: the best 3 to 5 stars for a fix at the next morning or evening twilight, spread round the horizon, with the altitude to preset on the sextant and the bearing to face; tap one to find it in AR Sky COASTAL NAVIGATION • Distance off a lighthouse, cliff or charted peak by its vertical sextant angle, to its waterline or to the sea horizon, with the tide below high water springs, your height of eye, the earth's curve and refraction (Bowditch's tables) • The vertical danger angle: the most the sextant may read to stay outside a clearing distance • Where a light rises or dips over the horizon: its distance from its height and yours, for a fix with one bearing SKY & EPHEMERIS • Sun, Moon, planets and the 57 navigational stars plus Polaris, computed to almanac accuracy (VSOP87 and Meeus algorithms, with nutation, aberration, refraction and ΔT) • Rise, set, transit and twilight times • Star Dome: the whole sky at a glance, with GoTo to any body • AR Sky: point your camera at the sky to see stars, planets, the Sun and Moon, all 88 constellations, classic Hevelius constellation art and the Milky Way. Search any star, planet or constellation and the app points you to it. AT SEA • Logbook of compass checks, sights and fixes, with CSV export • Dark, light and red night-vision themes that protect your night vision on the bridge Star data: HYG Database (CC BY-SA 4.0). Moon and Milky Way imagery: NASA Scientific Visualization Studio. Constellation art: Johannes Hevelius (1690), public domain. Compass Observation App is a training and cross-checking aid. It is not a substitute for official nautical publications or the ship's primary means of navigation; always verify with approved sources.