⚓ Compass Observation & Error Log
| Date / Time (UTC) |
DR Position Lat / Lon |
Ship Course | Observed Body | GHA / Dec | Alt (h) | True Brg Zn |
Repeater | Gyro Compass | Magnetic Compass | Officer | Del | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| GC | CC | GB | ΔG | MB | ΔM | Var | Dev (δ) | ✕ | ||||||||
Compass Observation Book & Error Log: Celestial Azimuths, Gyro Drift & Magnetic Deviation
Determining, recording, and monitoring compass errors is a mandatory watchkeeping duty governed by SOLAS Chapter V (Regulation 19), IMO Resolution A.382(X), and Section A-VIII/2 of the STCW Code. Despite the integration of GPS/GNSS and satellite heading devices, gyrocompass drift and magnetic compass deviation must be determined by observation at least once per navigational watch or whenever significant course alterations take place. Port State Control (PSC) officers and vetting inspectors (OCIMF SIRE 2.0) routinely audit the Compass Observation Book for gaps, unverified calculations, or anomalous deviation values.
The NavLib Compass Observation & Error Log automates celestial sight reduction and terrestrial verification in real time. Powered by an onboard Jean Meeus astronomical ephemeris engine, the platform instantly resolves true bearings (Zn) for the Sun, planets (Venus, Mars, Jupiter, Saturn), and 57 major navigation stars. The system evaluates gyro error (ΔG), total compass error (ΔM), and magnetic deviation (δ), logs observations into an indexed digital journal, and produces clean A4 landscape log sheets via an isolated print engine.
Frequently Asked Questions: Compass Observation & Error Logging (FAQ)
How frequently must compass errors be determined and recorded under SOLAS & STCW?
- After every major course alteration;
- Following heavy rolling or severe sea conditions;
- After loading or discharging magnetic cargoes (steel coils, scrap metal, iron ore);
- When returning to sea after a prolonged period in shipyard or drydock.
What are the operational tolerances for Gyro Error (ΔG) and Magnetic Deviation (δ)?
- Gyrocompass Error: must remain steady and should not exceed ±1.0° under normal operating conditions. Any sudden shift exceeding 1.0° indicates latitude/speed compensation errors, master gyro mechanical fault, or transmission failure.
- Magnetic Compass Deviation: should ideally remain within ±3.0° on any heading as referenced against the vessel's certified deviation table. A deviation difference greater than 3.0°–5.0° requires an official compass swing and adjustment by a licensed compass adjuster.
What mathematical formulas calculate Gyro Error, Total Error, and Deviation?
1. Gyro Error (ΔG):
ΔG = True Bearing (Zn) - Gyro Bearing (GB)(Positive result = East / High gyro; Negative result = West / Low gyro)
2. Total Magnetic Error (ΔM):
ΔM = True Bearing (Zn) - Magnetic/Compass Bearing (MB)3. Magnetic Deviation (δ):
δ = Total Error (ΔM) - Magnetic Variation (Var)(Applying the standard rule: Variation East is positive (+), Variation West is negative (-)).
Why does the tool generate warnings when the body's altitude exceeds 60° or falls below -0.5°?
- Altitude > 60°: When a body approaches zenith, slight tilts in the azimuth ring (pelorus vane) or minor roll motions produce severe horizontal angular discrepancies, rendering the bearing unreliable.
- Altitude < -0.5°: The body is physically below the astronomical horizon, signifying an erroneous observation time, incorrect DR coordinates, or an unobservable target.