Convert from spat to chain [Gunter, survey]

Unit Definition (spat)
The spat is an informal unit of distance formerly used by astronomers. The spat is equal to the terameter (Tm, or 10E12 meters). This is equivalent to about 6.6846 astronomical units. The origin of this usage is not clear; "spat" may be an acronym for "space unit" or "space-time".

Unit Definition (chain [Gunter, survey])
The chain is a unit of distance formerly used by surveyors. The traditional British surveyor's chain, also called Gunter's chain because it was introduced by the English mathematician Edmund Gunter (1581-1626) in 1620, is 4 rods long: that's equal to exactly 1/80 mile, 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). The traditional length of a cricket pitch is 1 chain. Gunter's chain has the useful property that an acre is exactly 10 square chains. The chain was divided into 100 links. American surveyors sometimes used a longer chain of 100 feet, known as the engineer's chain or Ramsden's chain. (However, Gunter's chain is also used in the U.S.; in fact, it is an important unit in the Public Lands Survey System.) In Texas, the vara chain of 2 varas (55.556 ft) was used in surveying Spanish land grants. In the metric world, surveyors often use a chain of 20 meters (65.617 ft).