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To aid navigation, this map shows small islands, shoals and reefs, maritime routes for use during monsoons or at particular times of the year, bathymetry (sea depth), tide information, shipwrecks, compass roses and landmark mountains at the coasts.

Navigation map of the southern South China Sea, with bathymetry (sea depth), islands, shoals and reefs marked. There are four maritime routes to China marked, with notes on what months or monsoon season to use them. Inset map of Gulf of Thailand.

Navigation map of the southern South China Sea, with bathymetry (sea depth), islands, shoals and reefs marked. There are four maritime routes to China marked, with notes on what months or monsoon season to use them. Inset map of Gulf of Thailand.

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.