The gaihōzu (外邦図), ‘maps of outer lands' produced by the Japanese Imperial Army between 1873 and 1945, represent one of the most comprehensive cartographic records of East and Inner Asia. Created during Japan's imperial expansion, the gaihōzu offer rich detail of territories beyond Japanese control. Despite their military origins, the gaihōzu now serve as geographical time capsules, preserving landscapes since transformed by modernization. This study documents the Great Mongolian Road, a major yet understudied east-west caravan route across Inner Asia. Through analysis of the Tōa Yochizu (東亞輿地圖) ‘Maps of East Asia’ series and field verification across 1200 km of southern Mongolia, we document the route's infrastructure for the first time. The gaihōzu capture not merely routes but complete support systems, including water sources, terrain features, and settlements vital for navigation and survival in these harsh arid environments. By mapping this historical corridor, these once-secret military documents provide valuable baseline data for historical geography, cultural heritage preservation, and environmental change assessment across the landscapes of Asia.

We would like to express our gratitude to Mr. Khasbaatar Khandmaa for his invaluable help with logistics and conducting local interviews. Our thanks also go to the Council of American Overseas Research Centers (CAORC) for providing the funding that made t

Gaihozu,Great Mongolian road,Historical cartography,Inner Asia,Mongolia,Silk roads,Water infrastructure

• Social and Economic Geography - Social Sciences
• History and Archeology - Humanities