Magnetic declination
A magnetometer points to the magnetic north, which differs from the north of a map (true north). The difference is the magnetic declination. With use_magnetometer of imu_orientation_estimator, set the declination in magnetic_declination_in_deg to reference the yaw to the true north. With 0, the reference is the magnetic north.
Mind the sign
magnetic_declination_in_deg is positive to the east (magnetic north east of true north). Japan has a west declination, so enter a negative value there.
The documents of the Geospatial Information Authority of Japan (GSI) count west as positive, the opposite sign. For example, 7°50′ west in GSI is entered as −7.83.
With the same measurement, changing the declination changes the yaw in the opposite direction: the yaw is positive counterclockwise, and an east declination is a clockwise offset.
How to find the value
- Japan: GSI magnetic charts. The calculation site gives the value for a place and a date.
- World: the calculators of the NOAA NCEI World Magnetic Model.
The declination changes with the place and moves slowly over the years. For a robot working in one area, a constant value is enough.
Typical values
Values for 2026.0 computed with the field model IGRF-14 (altitude 0 km, degrees, east positive; after 2025 the model is a prediction). They are a guide: check the value of your place on the sites above.
Japan:
| Place | Declination |
|---|---|
| Sapporo | −9.9 |
| Sendai | −8.5 |
| Tokyo | −7.9 |
| Nagoya | −8.1 |
| Suzuka | −8.1 |
| Osaka | −8.1 |
| Fukuoka | −8.0 |
| Naha | −5.8 |
World:
| Place | Declination |
|---|---|
| London | +1.1 |
| Paris | +2.0 |
| Berlin | +5.1 |
| New York | −12.5 |
| Los Angeles | +11.4 |
| São Paulo | −21.9 |
| Cape Town | −26.6 |
| Singapore | +0.2 |
| Shanghai | −6.5 |
| Sydney | +12.8 |
What the declination does not correct
The declination corrects only the offset of the Earth's field. Disturbances from nearby motors, steel and batteries remain. Model values can also differ from the real field by more than 1° in places (in Japan, up to 1.5° between the GSI chart and IGRF-14 over 300 points, 2020.0).