Fig. 6. Isoclinics (lines of equal magnetic inclination) and isodynamics (lines of equal magnetic field intensity) in the Indian Ocean, along the east coast of Africa. Isoclinics run approximately east–west in this region and are shown in 1° contours. Isodynamics run approximately north–south and are represented in 1000 nT increments. Thus, in this geographic area, the two sets of isolines form a nonorthogonal grid that birds, sea turtles, fish or other animals might, in principle, exploit as a kind of bicoordinate magnetic map. Whether any animal does this is not known. (Modified from Lohmann et al., 1999.) Isolines were derived for the year 1995 from the International Geomagnetic Reference Field (IGRF) 1995 model.