Gemma Frisius (1508-1555)

Of German origin, Gemma Frisius was the most prominent astronomer, mathematician and surveyor of his time. He taught Mercator at his School of Geography in Louvain and contributed greatly to the development of cartography. He invented a form of cross-staff for use in astronomy and published 'A Method for delineating places' in which he described the principle of triangulation in map making. He was also the first to propose the use of portable clocks for determining longitude.