The gaihozu, '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 gaihozu offer rich detail of territories beyond Japanese control. Despite their military origins, the gaihozu 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 Toa Yochizu '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 gaihozu 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. (c) 2025 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.