The Nine Chapters on the Mathematical Art, Vols. 1-3

...their numbers, keeping them in the mind to illuminate the beauty of worldly techniques. I have thus created the Chong Cha and provided it with annotations to investigate the intentions of the ancients. It is appended below the Gou Gu section. For measuring heights, use the double gnomon; for measuring depths, use the cumulative square; for isolated objects, use three-point observation; for those that are separated and require lateral measurement, use four-point observation. By applying these categories to grow the system, even the dark, the distant, and the strangely hidden will have nowhere to remain concealed. I hope the scholarly gentleman will examine this in detail.