William Roy was a polymath and a visionary. His work established the path that would lead to the formation of the Ordnance Survey and to all of the paper-based and digital mapping products that we use today. His story - very much one of the Enlightenment - demonstrates how one man's curiosity and diligence enabled him to excel across a diverse range of topics: military reconnaissance and intelligence; the lessons that could be learned from the past about the tactical use of landscape; the science of determining the height of mountains; and the development of a meticulous methodology to achieve an unprecedented accuracy in topographical measurement. In this biography, Humphrey Welfare uncovers the career and activities of this important figure, and in doing so paints a vivid picture of the inner complexities of 18^th-century Britain.