What is "gunter's chain"

Gunter's chain \Gun"ter's chain`\ (Surveying) The chain ordinarily used in measuring land. See Chain, n., 4, and Gunter's scale.

Gunter's chain or the surveyor's chain (also known as Gunter’s measurement or surveyor’s measurement) is a distance measuring device used for land survey. It was designed and introduced in 1620 by English clergyman and mathematician Edmund Gunter (1581–1626) long before the development of the theodolite and other more sophisticated equipment, enabling plots of land to be accurately surveyed and plotted, for legal and commercial purposes.

Gunter used an actual measuring chain of 100 links. These, the chain and the link, have become units of their own.