The Great Wall is a window for the world to understand China. In 1687, Jean-Francois Gerbillon, a Frenchman, came to China and wrote the first professional report on the Great Wall based on the field investigation.
Two years later, the Kangxi Emperor, the third emperor of the Qing dynasty, ordered western missionaries in China to draw a map, which charted more than 300 passes along the Great Wall by triangulation, marking the beginning of accurate measurement of the Great Wall. It has been more than 300 years since the two achievements were made. Since then, explorers and scholars of western countries have carried out relevant researches.