一行
The astronomer Yi Xing (683–727) of the Tang Dynasty built 20 standardized gnomons spread out over China to measure the equation of time dependent on the geographical location. Following a proposition of Liu Zhuo from 604 AD, 10 of these were aligned along the meridian 114° east of Greenwich from Central Asia down to Vietnam in order to determine the circumference of the Earth and deviations from a perfect sphere. One of these 10 observatories was situated at Gaocheng. The observations were used to establish the Da Yan calendar.