唐代開元年間，一行在其《大衍曆》（724）中構造了世界上第一張正切函數表， 與此同時，他發起並領導了中國歷史上第一次大規模的天文大地測量，其結果歸算出了地球 子午線一度所對應的長度。本文的主要目的是通過具體的計算，分析並探討一行的正切函數 表與史籍中記錄的與這次測量有關的大量數據的關係。根據《高麗史》中所載唐代《宣明曆》 （892）的有關算法，重構了一行的正切函數表。通過對這次測量中全部十個地點的所有數據 的計算發現，史籍上記錄的測定結果，如「定春秋分晷影」等，基本上都是利用一行的函數 表根據冬、夏至晷影而歸算出來的。一行的正切函數表對唐代天體測量學的理論方法產生了 很大的影響。

　　　　　In 724 AD the Buddhist monk Yixing (I-Hsing) produced the world's first tangent table in his Dayan Calendar. At that time, he also organized and led the first astronomical geodetic survey in the history of China. As a result of this survey, the length of a meridian line in one du (1 du=(360/365.2565)°) was deduced. The main purpose of this paper is to analyze and explore the relationship between Yixing's tangent table and astronomical geodetic survey. Based on historical material in the History of Korea, I reconstruct the tangent table in Yixing's Dayan Calendar. Through examination of data from the above survey in historicalrecords, I also determine that all related figures, such as the length of equinoctial shadow, were calculated using Yixing's tangent table and measurements of the length of summer or winter solstice shadow. I conclude that Yixing's tangent table had a strong influence on the theory and methodology of celestial surveying in the Tang dynasty.