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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).

Hand-drawn portolan (nautical) chart, featuring a rhumbline network (the web of interconnected lines), bathymetry (sea depth), shoals and reefs, names of coastal settlements, and an elevation (side view) of a mountain at the north tip of Borneo.

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.