Fig. 3

Map of the reference field, which consists of magnetic isoclines fitted to the distribution of magnetic intensities measured by observatories located worldwide. Geomagnetic geolocations are most accurate in regions where the magnetic field intensity gradient is steep and runs north–south and thus where the lines of equal magnetic field intensity are dense and run approximately east–west. There are generally two locations (solutions) for each north–south line. In this case, the drifter is more likely to be at 38°N than 85°S, which is on the Antarctic Peninsula (see dashed red lines in Fig. 3). Note that the geolocation (red circles) for the drifter would be accurate because the dipole gradient is steep in the western North Atlantic Ocean