Magnetic Field Logger: Compass Variation Tracking

The Earth’s magnetic field is constantly changing — declination shifts by arcminutes per year, and geomagnetic storms from solar activity can cause sudden variations. A magnetic field logger built with an Arduino and digital magnetometer records these changes, providing data useful for navigation calibration, geomagnetic research, and detecting solar storm effects. This project is especially relevant in India where declination varies from -2° to +3° across the country.

Understanding Earth’s Magnetic Field

Earth’s magnetic field has both a steady component (the geodynamo) and variations from solar activity, crustal anomalies, and secular drift. The field at any location is described by declination (angle from true north), inclination (dip angle), and total intensity (measured in nanotesla, nT).

In India, the Indian Institute of Geomagnetism (IIG) operates magnetic observatories at Alibag, Hyderabad, and several other locations. Citizen science stations can supplement these, especially during geomagnetic storms when high spatial resolution data is valuable.

Magnetometer Sensors for Arduino

Two magnetometer modules are commonly available for Arduino projects:

• HMC5883L / QMC5883L — 3-axis magnetometer, ±8 gauss range, 12-bit resolution. I2C interface. The QMC5883L is the current-production replacement for the discontinued HMC5883L. Cost: ₹150-250.
• MLX90393 — Higher resolution, ±50 gauss range, SPI/I2C interface. Better for measuring both Earth’s field and local anomalies. Cost: ₹400-600.

Building a Magnetic Field Data Logger

The data logger records magnetic field components (X, Y, Z) every second and stores to an SD card:

• Arduino Uno or ESP32
• QMC5883L or HMC5883L magnetometer module
• DS3231 RTC module for accurate timestamps
• SD card module for data storage
• OLED display for real-time readout

Place the sensor away from all ferromagnetic materials (iron, steel, nickel). Even a steel screw 10 cm away will distort readings. Use a wooden or plastic mounting platform.

Compass Declination Mapping in India

Magnetic declination in India ranges from about -2° in the northeast to +3° in the northwest. The Indian Survey publishes isogonic maps updated every 5 years. With your logger, you can measure local declination by pointing a laser level at true north (from GPS or Polaris observation) and reading the magnetometer.

Declination changes about 0.1° per year in India. A 5-year measurement series from your station would clearly show this drift — an educational and scientifically valuable dataset.

Detecting Geomagnetic Storms

Geomagnetic storms from solar coronal mass ejections (CMEs) cause rapid fluctuations in the magnetic field — up to 500 nT variations over hours. Your logger will detect these as anomalous readings compared to the quiet-day baseline.

Set up an alert when the rate of change exceeds 50 nT/hour. During the March 2024 solar storm, Indian magnetometers recorded variations visible even to hobbyist-grade QMC5883L sensors.

Applications in Navigation and Surveying

Practical applications of your magnetic field logger:

• Calibrating compass modules for drone and robot navigation
• Surveying for buried metallic objects (pipelines, archaeological artefacts)
• Validating Vastu measurements (Vastu Shastra relies on magnetic north, which differs from true north)
• Detecting ferromagnetic anomalies in building construction

Interference Sources and Shielding

Common interference sources that affect readings:

• Power lines (50 Hz hum, ±10-50 nT at 10m distance)
• Steel reinforcement in concrete buildings
• Electronic devices (phones, laptops) within 1 metre
• Moving vehicles

For clean data, deploy the sensor in an open area at least 10 metres from buildings and power lines. A mu-metal shield around the sensor can reduce external interference but also attenuates the Earth’s field — avoid it for geomagnetic measurements.

Long-Term Geomagnetic Trends

The Earth’s magnetic field has been weakening over the past 200 years. The South Atlantic Anomaly — a region of weak field — has been expanding. Your multi-year dataset contributes to understanding these long-term trends at your specific location.