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Nautical charts like this are detailed around the coasts and on the seas (e.g. river mouths, islands, shoals and reefs) while the interiors are blank. The web of lines is a rhumbline network, while the numbers indicate sea depth (bathymetry).

Maritime routes had to be adjusted in response to the changing monsoon seasons, as is seen on this late 18th century map of the Indian Ocean. Drawings of the heads of children blowing—called ‘wind-heads’—are used to indicate wind direction.

Two maps of the Indian Ocean showing currents (represented by arrows) and winds (represented by letters A to F) during the monsoon seasons. Three routes—one old and two new—from the Isle de France (Mauritius) east to the Strait of Malacca are shown.