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Mont-Saint-Michel Light Show, France, 1998

"Light reflected from nylon thread near Mont-Saint-Michel illustrates triangulation, the method Jacques Cassini and his son César-François used to survey 18th-century France. Starting from the Paris meridian, they sighted off high landmarks to accurately measure distance. At Mont-Saint-Michel, the lines of sight from four vantage points met in the apexes of three triangles at the steeple of the medieval abbey. By 1744 the Cassinis had crisscrossed the country with 400 such triangles."

—From "Revolution in Mapping," February 1998, National Geographic magazine

Photograph by Bob Sacha