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Gemma Frisius (1508-1555), born as Regnier Gemma, Dutch mathematician, cartographer and instrument maker. Frisius set up a workshop for making globes and mathematical instruments while he was a student at Leuven, Belgium. His reputation spread, particularly when the astronomer Tycho Brahe praised the quality and accuracy of his instruments. In 1533 Frisius developed trigonomic surveying, still in use today, which uses the method of triangulation to locate places. He was also the first to describe how an accurate clock could be used to determine longitude, although the first person to actually build such a timepiece was 18th century clockmaker John Harrison.
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