Geolocation with a magnetic sensor

Many migratory animals use spatial variation in the Earth's magnetic field for orientation and navigation. This raises the possibility of exploiting these same cues to geolocate and track migratory animals throughout their annual journeys. Magnetic-based geolocation could be particularly valuable for small aerial and aquatic species, for which the most precise pressure-based geolocation cannot be applied. We developed a novel geolocation method, implemented in the R package GeoMagR (https://geopressure.org/GeoMagR/) that uses three-axis magnetic field measurements from lightweight multi-sensor tags. The workflow consists of: (i) tilt compensation and magnetic calibration; (ii) extraction of time series of magnetic intensity and inclination; and (iii) generation of spatial likelihood maps using the World Magnetic Model (WMM). We evaluated the method using field data to assess achievable spatial precision and potential complementarity with other geolocation techniques. Magnetic geolocation achieved a typical spatial accuracy of ~150 km in latitude but provided little constraint in longitude. When combined with light-based geolocation, which offers high longitudinal accuracy but poor latitudinal resolution, the integration markedly improved overall position estimates. Magnetic geolocation is a viable complementary tool for tracking small avian species, particularly in contexts where pressure-based methods cannot be used. By independently resolving latitude, it enhances spatial inference when integrated with other geolocation data sources. This processing of 3D magnetic and acceleration data from geolocators also extends their use beyond positioning, enabling applications in studies of behaviour and navigation.