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Map of the coast of Tsiompa and Couchinchina (Vietnam) designed for navigation, with a rhumbline network and bathymetry (sea depth). The large area marked with crosses represents the Paracel Islands. The modern city of Da Nang is at the far right.

Early navigation map of the South China Sea. The web of lines is a rhumbline network, while the numbers indicate sea depth (bathymetry), both used to aid navigation. A route around the Gulf of Thailand is marked with text and the dates 1643 and 1644.