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Map of the Riouw (Riau) Residency, Sumatra. The border between Riau and the island’s west coast is marked in red ‘as far as it is known [voor zoo verre die... bekend is]’, ending abruptly in the unexplored mountains near Lake Toba, northern Sumatra.

Map of the Dutch colonial administrative region (residency) of Riouw (Riau) focusing on the islands and sea passages, with bathymetry (sea depth) marked. (From ‘Algemeene Atlas van Nederlandsche Indie [General Atlas of the Dutch East Indies]’.)