Travel north and the pole star stands higher in the sky; travel south and it stands lower. Set up a gnomon at noon on the same day and the shadow differs in length from place to place. Setting these two against the road distance between two places north and south shows how far one must go for the height in the sky to change by one degree.
In 724 (Kaiyuan 12 of the Tang), Yi Xing and Nangong Yue organised measurements at sites from north to south, taking both shadows and the height of the pole, and obtained the length of one degree of the meridian. Measured figures refuted an older theory.
In the Yuan, Guo Shoujing directed the Survey of the Four Seas and set up twenty-seven observation sites across the realm.
The Kangxi Emperor ordered Chinese and Western surveyors together to measure latitude and longitude across the country, and the length of the li was fixed by the length of one degree of the meridian, producing the Huangyu Quanlan Tu.
Meridian Measurement is the class in the chapter on Cosmos on surveying the land: measuring shadows and the height of the pole at many sites from north to south to find the length of one degree of the meridian. In 724 (Kaiyuan 12 of the Tang) Yi Xing and Nangong Yue organised such a survey, the Yuan had Guo Shoujing's Survey of the Four Seas, and under Kangxi the Qing measured latitude and longitude across the country and produced the Huangyu Quanlan Tu.
