Drawing from an assortment of government reports, contemporary publications, and cartographic materials, this article examines the triangulation survey conducted by the Japanese government-general in Korea from 1910 to 1918. In addition to elucidating the mapmaking process, it explores the ways in which the triangulation survey both reflected and promoted Japan's colonial authority in Korea and abroad. By turns, the author provides a broad sketch of the planning and implementation of the survey, considers the tools and techniques that enabled it, traces the progress of the triangulation enterprise, and dwells on the legacy and limitations of the maps brought about by the triangulation survey.